Subdivision from group including FBI agents (7)
I believe the answer is:
segment
'subdivision' is the definition.
(I've seen this in another clue)
'group including fbi agents' is the wordplay.
'group' becomes 'set' (both can mean a collection).
'including' is an insertion indicator.
'fbi agents' becomes 'g-men' ('G-man' can be a synonym of 'fbi agent').
'set' placed around 'gmen' is 'SEGMENT'.
'from' acts as a link.
(Other definitions for segment that I've seen before include "Piece of an orange" , "Separate" , "Gets men with part cut off" , "split up" , "Part of citrus fruit" .)