About the only justification for "McClain's Law," a new NBC cop series, is that it may one day become ripe parody material for those wackily wicked funsters of "SCTV," which ironically enough is becoming network TV's haven of sanity. The two-hour pilot for "McClain," which brings James Arness drowsily back to television, can be seen tonight at 9 on Channel 4, with doom written all over it.

Arness plays a crusty cop named Jim McClain -- surely a homage to "Big Jim McLain," the 1952 film in which John Wayne played a Commie-hunter and Arness was a fellow agent snuffed by the Reds (it was Wayne who suggested Arness for the starring role in "Gunsmoke"). And that bit of movie trivia is about the extent of distinction for this recklessly humdrum cop opera, which leaves no survivors in its wake.

Actually, "Old Jim McClain" would have been a more appropriate title. While still a bracing presence of sorts, and radiating residual good will earned in days of yore, Arness looks considerably more aged than the 52-year-old detective he is supposed to be playing, thus reinforcing NBC's image as the geriatric network. There is something slightly rib-tickling about Arness's appearance; it isn't so much the ruggedly weathered Arness face as it is the fact that one of Virginia Graham's old hairdos seems to have landed on top of it.

He looked better as The Thing.

Not even Arness in his post-"Thing" prime could have made credibility out of writer Eric Bercovici's painfully primitive, almost facetious screenplay, however. When his longtime fishing partner is robbed and killed after flashing $10,000 in cash around a bar, McClain scowls, "It just doesn't figure" (yes it does) and vows of the killers, "I'm sure as hell gonna gettum."

He decides to return to active duty on the San Pedro police department, where he earlier served as a detective, but of course he is met by testy resistance from a surly young lieutenant who says things like, "In your day, which was before my time," and that ever-popular rhetorical thunderer, "Do I make myself clear?"

Soon big Jim is teamed with a blandly blond young partner fresh from a hairspray commercial, and listening to cronies say things like, "Just how long do you think you'd last out there on the street, huh?" Naturally the reply is, "One way to find out." And in the final scene, after McClain shoots the guy what shot his pal, the young partner says, "You said you'd get him," and big Jim says, "Yeah." The shrugs and grunts may have worked on "Gunsmoke," but San Pedro is no Dodge City, and Jim McClain, big or not, is no Matt Dillon.