Belligerent, in British slang (5)
I believe the answer is:
aggro
'belligerent in british slang' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aggro that I've seen before include "Difficulties, trouble" , "Potential violence" , "Antisocial behaviour" , "belligerence" , "Violence - difficulties" .)