Part of series involving Federal agents (7)
I believe the answer is:
segment
'part' is the definition.
(segment is a kind of part)
'series involving federal agents' is the wordplay.
'series' becomes 'set' (I've seen this before).
'involving' means one lot of letters goes inside another.
'federal agents' becomes 'gmen' (old-fashioned informal term for the FBI).
'set' enclosing 'gmen' is 'SEGMENT'.
'of' acts as a link.
(Other definitions for segment that I've seen before include "Part separable from other parts, say of orange" , "Division" , "Part of citrus fruit" , "split up" , "Part, piece" .)