Draft for American screenplay supporting prisoner (9)
I believe the answer is:
conscript
'draft' is the definition.
(conscripting is a kind of drafting)
'american screenplay supporting prisoner' is the wordplay.
'american screenplay' becomes 'script' (US term for screenplay).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'prisoner' becomes 'con' (abbreviation for 'convict').
'script' after 'con' is 'CONSCRIPT'.
'for' is the link.
(Other definitions for conscript that I've seen before include "One who hasn't volunteered" , "One drafted into the forces" , "One compulsorily enlisted" , "One enrolled" , "draft on Broadway?" .)