Nasty film featuring Republican brawl (5)
I believe the answer is:
scrum
'brawl' is the definition.
(I've seen this before)
'nasty film featuring republican' is the wordplay.
'nasty film' becomes 'scum' (I've seen this in another clue).
'featuring' means one lot of letters goes inside another.
'republican' becomes 'R' (abbreviation in US politics).
'scum' enclosing 'r' is 'SCRUM'.
(Other definitions for scrum that I've seen before include "group of forwards" , "Rugby huddle" , "Rugby-players crouch here" , "Disorderly struggle" , "crush" .)