If there were a position for "clown prince of jazz," it might well go to Clark Terry's alter ego, the aptly named Mumbles. When Terry launches into his unique combination of scat singing and Sid Caesar-esque double-talk, you laugh enough--no matter how often you've heard it--to overlook the fact that he's actually making plenty of musical (as opposed to syntactical) sense. Terry's wit extends to his trumpet and flugelhorn playing, too; but when he's playing Mumbles, his prowess is too overpowering to miss. An early model for Miles Davis, a mainstay of the great Duke Ellington bands of the 1950s, and at this point a certifiable jazz mandarin, Terry balances art and entertainment in a manner unmatched by most of his juniors. So he's found a perfect partner in saxophonist Red Holloway, another unabashed swinger who has mastered showmanship as well as the complexities of improvisation. They'll front the Willie Pickens Trio and, on the 31st, make room for a quartet of trumpet prodigies (including the fine Chicago player Rod McGaha) to ring in the new year--or at least mumble away the old one. Next Friday, December 28, through Monday, December 31, Joe Segal's Jazz Showcase, Blackstone Hotel, 636 S. Michigan; 427-4300.