Part in sitcom for terrible dummy (9)
I believe the answer is:
comforter
'dummy' is the definition.
(I've seen this before)
'part in sitcom for terrible' is the wordplay.
'part in' indicates a hidden word.
'COMFORTER' is hidden in the letters of 'sitcom for terrible'.
(Other definitions for comforter that I've seen before include "Job didn't need such as this" , "Baby's dummy" , "one that brings relief" .)