Split group infiltrated by Feds (7)
I believe the answer is:
segment
'split' is the definition.
(thesaurus)
'group infiltrated by feds' is the wordplay.
'group' becomes 'set' (both can mean a collection).
'infiltrated by' means one lot of letters goes inside another.
'feds' becomes 'g-men' (I've seen this in another clue).
'set' enclosing 'gmen' is 'SEGMENT'.
(Other definitions for segment that I've seen before include "Part of, eg, an orange" , "split up" , "Piece of an orange" , "(Divided-up) part" , "Section of a circle" .)