An entertaining slice of the American theatrical past comes to us tonight by way of public TV's "Theatre in America" series. It's a spy drama called "Secret Service," revived with a hatful of winsome verve by New York's Phoenix Theatre group, the nation's oldest repertory company. It airs at 9 o'clock on Channels 26 and 22.

Actor-manager William Gillette wrote the play in 1895, and when he starred as Capt. Thorne in the New York production a few years later it made him a national celebrity. Thorne is a Union undercover agent who poses as a Confederate officer (the setting is Richmond, 1864) and falls in love with a Southern belle. His successful juggling of these conflicting loyalities is the basis of the lively, intricate plot.

The Phoenix cast, under the direction of Daniel Freudenberger and Peter Levin, manages the adroit feat of treating the script simultaneously as earnest melodrama and quaint relic. The characterizations are straight-faced, but consciously overlaid with the artifices of turn-of-the-century stylization. The result is that the performance can be enjoyed on two levels at once - as a respectfully restored period piece, and as a memento of the stage conventions of a vanished erra. No one twirls handlebar mustaches; nothing is burlesqued. But you know that the actors know that you know they're not taking it seriously.

Particularly engaging are John Lithgow (the villian of "Obsession") as Thorne, Meryl Streep as his plantation sweetheart, Marybeth Hurt as her flibbertigibberty friend, and Charles Kimbrough as the Conderate heavy.