Buster —, film comedian (6)
I believe the answer is:
keaton
(Other definitions for keaton that I've seen before include "Buster ...... was star comic of silent screen" , "Buster, silent film star" , "Buster . . . . . ., early film comedian" , "Buster --, comic actor" , "Buster ...... was stolid star of silent screen" .)