Although born in New Hampshire, Clapp studied drama in Canada and first gained experience as an actor in regional Canadian productions of Shakespeare plays and other works. He made an impressive feature debut as Chip, a senator's aide, in John Sayles' fine "The Return of the Secaucus Seven" (1980), which was later copied in Hollywood as "The Big Chill". Although Clapp would later work with Sayles as a union buster in "Matewan" (1987), as a quick-tempered baseball catcher in "Eight Men Out" (1988) and as a local activist in "Sunshine State" (2002), his other feature credits have been modest, most prominent among them a large role in the downbeat, Canadian-made family portrait, "Termini Station" (1989).

Beginning in TV with a small regular role on the USA Network's sitcom about a supermarket manager, "Check It Out!" (1985-88), Clapp began to perform in many and varied TV-movies and miniseries. Most typically cast as military men, police detectives and working-class types, he played roles in "The Right of the People" (1986), "Blind Faith" (1990), "Fever" (1991), "Bonds of Love" (1993) and, alongside fellow Conway NH native John Shea, in "Small Sacrifices" (1989). He got his breakthrough role as Medavoy in 1993, and the role allowed Clapp to gradually flesh out his character beyond his initial stamering insecurity, seeing him through divorce, a romance with a gorgeous secretary, get-rich-quick moonlighting gigs and a string of new partners. He frequently provided a perfect foil for the hard-edged Sipowicz, and the actor's ability to covney Medavoy's failure to measure up to his colleague's tough demeanor was on the hallmarks of the long-running series.

Clapp's expanded profile led to supporting roles in such features as "Carrie 2: The Rage" (1999), "Rules of Engagement" (2000) and "Moonlight Mile" (2002) and a steady slate of made-for-television movies.