Film about British dissident (5)
I believe the answer is:
rebel
'dissident' is the definition.
(I know that rebel is a type of nonconformist)
'film about british' is the wordplay.
'film' becomes 'reel' (reel is a kind of film).
'about' indicates putting letters inside.
'british' becomes 'b' (abbreviation e.g. in BBC).
'reel' enclosing 'b' is 'REBEL'.
(Other definitions for rebel that I've seen before include "Do things differently" , "Insurgent, mutineer" , "Authority fighter" , "Rise up (without a cause?)" , "Oppose authority" .)