This lean, often bearded, character player of stage and screen since the 1970s gained some measure of celebrity as the cold, cunning Miles Dentrel on the acclaimed dramatic series "thirtysomething" (ABC). As the resident yuppie scum from 1989-1991, Clennon portrayed the calculating character who seemed to represent the fears and reservations of the show's more sympathetic figures. That his prior stints as a TV regular--"Rafferty" (CBS, 1977), a medical drama and "Park Place" (CBS, 1981), a short-lived legal sitcom--had him playing a surgeon and an eager, idealistic legal aide lawyer,