A rare opportunity to see the Clash, the influential British quartet that fused white punk and black reggae into a political and socially complusive rock energy, is the reason to see "Rude Boy" -- a semi-documentary British film which shows tonight only at the Embassy Circle as part of a special anti-draft benefit.

The film follows around the Clash, and its vague connecting story line follows loutish 19-year-old Ray Gange, who insinuates himself as a roadie for the group as it tours the British Isles, rehearsing, relaxing, recording and ultimately regrouping in the face of growing success. Like much punk music, "Rude Boy's" urgency is diminished by its lack of consistent focus, but the straightforward, non-sensational portrait of the band and its fans should do much to dispel media-misdirected notions of punk esthetics.

The beautifully photographed film succeeds as a concert film and as a road film, which is probably the last thing directors Jack Hazan and David Mingay had in mind. Any of the half-dozen set-ups and breakdowns for concerts tells one more about being on tour than "Roadie" ever did. Gange, as the dullard hanging on, is an equally familiar reality, while many of the jump scenes between performances grimly capture the gray world of rehearsals and recording studios.

There's one beautifully played scene in which the Clash's lead singer, Joe Strummer, is alone in a half-lit studio, instrumental tracks barely discernible through the headphones. Strummer starts to sway and breathe hard as he approaches his place in the song; when he excitedly brusts into the lyrics of "All the Young Punks," it doesn't matter that he's off-key and barely on the melody line. The song capsulizes the raw emotional power evident in the film's other 20 Dolbyized songs, which include "Police and Thieves," "London's Burning," Complete Control" and "White Riot," one of the key songs in the punk movement and a prophetic glimpse of recent disturbances in London.

There are problems with "Rude Boy" -- much of the dialect is unintelligible, boom mikes drop into view several times, the political rhetoric is as naive as it is earnest. The political scenarios will seem obscure to many American viewers. But with persistent rumors of a breakup and dim prospects of seeing the Clash live again, "Rude Boy" is a most promising alternative. The film will be shown at 7 and 9:30 p.m. tonight only.