Assist in a B movie? (4)
I believe the answer is:
abet
'assist' is the definition.
(abetting is a kind of assisting)
'a b movie?' is the wordplay.
'movie?' becomes 'et'.
'a'+'b'+'et'='ABET'
'in' is the link.
(Other definitions for abet that I've seen before include "Encourage (wrongdoing)" , "Incite, help" , "Help in wrongdoing" , "Egg on" , "Assist (in doing wrong?)" .)