Split up FBI agents in group (7)
I believe the answer is:
segment
'split up' is the definition.
(thesaurus)
'fbi agents in group' is the wordplay.
'fbi agents' becomes 'gmen' ('G-man' can be a synonym of 'fbi agent').
'in' is an insertion indicator.
'group' becomes 'set' (set is a kind of group).
'gmen' placed inside 'set' is 'SEGMENT'.
(Other definitions for segment that I've seen before include "One of several parts of a whole" , "Part of, eg, an orange" , "One part of a whole" , "Part, piece" , "(Divided-up) part" .)