One of the tidier ironies of our time is embodied in the exploitation movie about exploitaiton. "City in Fear," a three-hour ABC production tomorrow at 9 on Channel 7, tsk-tsks over a ruthless newspaper's exploitation of a "Son of Sam"-type killer in order to gain circulation.

But this compelling, well-made movie about a homicidal maniac on a rampage can also be seen as an attempt to gain ratings with the same kind of sensationalism. If Albert Ruben's screenplay actually were about ideas -- right-to-know and freedom-of-press issues -- there would be no real need to depict the killings at all.

But whoa there, this is television, right? So there are no fewer than six shootings, most of them fatal, all of them of beautiful girls, within the first 90 minutes. They are discreetly edited and not gory, but one involves a coluptous young woman in a one-piece, tight-fitting red swimsuit. She reclines seductively on the beach for the benefit of the psycho and the camera.

Whatever the phyocrisy inherent in the project, at least Ruben does bring to it a certain media consciousness about celebrity criminals and the disappearing thin line between fame and notoriety. And director Alan Smithee dispatches cinematic cliches very effectively.

In addition, Robert Vaughn is scintillating and insinuating in his usual suave-oily way as a Rupert Murdoch sort of publisher, and David Janssen is unusually poignant and powerful in this, his last major TV role.

Janssen may have had extremely limited range as an actor, but acting for television is largely a matter of retaining tight controls and using the small gesture shrewdly. He does this faultlessly in several scenes, playing a frazzled world-weary old reporter calle d out of retirement to help the Los Angeles Sun shine again with lurid accounts of the killer's spree.

Vaughn and Janssen are especially punchy in their scenes together, including one near the film's end in which Janssen almost literally punches the publisher, stopping himself just short of a quick trip to the sidewalk. But Janssen's finest moment is probably a lunch with his literary agent, whom he begs to rescue him from grubby daily journalism with a juicy book contract. Ouch.

The last we see of David Janssen, the reporter he plays has watched his dreams of wealth die with the psycho on a disco floor, and he waves a little and, in context, poitnant farewell to the publisher's wife (the easy-to-take Susan Sullivan), who had shared his side in the fight against the publisher's recklessness.

It isn't a good movie, exactly, and the way it tosses topics around is pretty-cheap -- except by the prevailing TV standard. There are a lot of unlikely utterances from the mouths of the publisher, his editors, reporters and cops, but to Ruben's credit, there are some strikingly likely ones as well. "City in Fear" is immensely and intensively watchable. "Prime Time Saturday'

Two outstanding pieces of reportage surface tonight on NBC's much-troubled, underpromoted and consistently enterprising "Prime Time Saturday," with Tom Snyder, at 10 on Channel 4.

The first of these, scheduled to open the program, concerns the efficacy of marijuana and the related drug THC in offsetting adverse effects, chiefly nausea, of chemotherapy used to treat some cancer patients. The problem is, as Jessica Savitch reports, that the government is hemming, hawing and dawdling over allowing doctors to prescribe the fun drug to patients who could benefit from it.

Some government-controlled pot is being grown and dispensed for such use, Savitch reports, and is even rolled into cigarettes for easy consumption. "Government joints have a consistent potency" that street reefers lack, Savitch notes.

We also meet, from the neck down, a wealthy Seattle pot farmer who donates part of his crop for medicinal use by cancer patients. "Marijuana has been very good to me," he explains. "I have a social conscience like anyone else." High Times magazine will have a high time with this one.

Not mentioned is a thorny issue of the same type -- the advocacy of prescribing heroin to treat extreme pain in some cancer cases, a practice permitted in England but only in the discussion stages here. But the report, produced by Vernon Hixson, is crisp and straightforward.

Also scheduled for tonight is producer Robert Lissit's dossier on the incredibly ill-fated Air Force C-5 cargo plane, which looks adorably goony-birdish in superb video photography but which correspondent John Dancy says represents a "dismal job" by Lockheed, which got the contract to build it.

When the wings of the C-5 proved defective, Lockheed also got the $1 billion contract to modify them, Dancy reports; this overhaul won't be completed, and the C-5 completely shipshape, until 1987. Now another place, the C-X Strategic Air Lifter, is on the drawing boards and "the same promises" are being made for it as were made, but not kept on the C-5, Dancy says. Good stuff.

Not available for preview was a third segment on an ultra-Christian and highly victorious basketball team. "Prime Time Saturday" may lack the hype and hoopla of "60 Minutes," but some of its pieces are more sophisticated and more subtle. It's too bad NBC does nothing to support it other than -- almost begrudgingly -- putting it on the air.