Film about British insurgent (5)
I believe the answer is:
rebel
'insurgent' 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' placed around 'b' is 'REBEL'.
(Other definitions for rebel that I've seen before include "Mutineer, one who opposes authority" , "Freedom fighter?" , "One who won't comply" , "One rising in opposition" , "Eg, Cade or Hereward" .)