Separate FBI agents in series (7)
I believe the answer is:
segment
'separate' is the definition.
(segmenting is a kind of separating)
'fbi agents in series' is the wordplay.
'fbi agents' becomes 'gmen' ('G-man' can be a synonym of 'fbi agent').
'in' is an insertion indicator.
'series' becomes 'set' (I've seen this before).
'gmen' put into 'set' is 'SEGMENT'.
(Other definitions for segment that I've seen before include "Part of citrus fruit" , "Portion" , "Separate" , "Piece of an orange" , "Part, piece" .)