Subject to film? (5)
I believe the answer is:
topic
'subject' is the definition.
(as in a topic for discussion)
'to film?' is the wordplay.
'film?' becomes 'pic' (short for picture).
'to'+'pic'='TOPIC'
(Other definitions for topic that I've seen before include "Subject (for conversation, etc)" , "Talking point" , "Subject (of, eg, speech)" , "Theme (of, eg, a talk)" , "Subject (of a talk, etc)" .)