Not all an actor may play (4)
I believe the answer is:
part
'not all an' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for part that I've seen before include "Region" , "Portion, segment" , "Piece; divide" , "Some - theatrical role" , "Divide; role" .)