Fixed clothing FBI agents cut up (7)
I believe the answer is:
segment
'cut up' is the definition.
(thesaurus)
'fixed clothing fbi agents' is the wordplay.
'fixed' becomes 'set' (both can mean decided or determined).
'clothing' is an insertion indicator (some letters cover up or clothe others).
'fbi agents' becomes 'g-men' ('G-man' can be a synonym of 'fbi agent').
'set' enclosing 'gmen' is 'SEGMENT'.
(Other definitions for segment that I've seen before include "Separate" , "Gets men with part cut off" , "Division" , "One of several parts of a whole" , "(Divided-up) part" .)