Set up part in movie by American, possibly (5)
I believe the answer is:
maybe
'possibly' is the definition.
('maybe' can be a synonym of 'possibly')
'set up part in movie by american' is the wordplay.
'set up' says the letters should be written backwards.
'part in' indicates a hidden word.
'ebyam' can be found hidden inside 'movie by american'.
'ebyam' in reverse letter order is 'MAYBE'.
(Other definitions for maybe that I've seen before include "Perhaps, might happen" , "Perhaps, possibly" .)