Film about British mutineer (5)
I believe the answer is:
rebel
'mutineer' is the definition.
(synonyms)
'film about british' is the wordplay.
'film' becomes 'reel' (reel is a kind of film).
'about' means one lot of letters goes inside another.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'reel' enclosing 'b' is 'REBEL'.
(Other definitions for rebel that I've seen before include "Rise up (without a cause?)" , "He resists authority" , "Kick over the traces" , "Bolshy" , "Dissident" .)