Regulate boxing agents, in part (7)
I believe the answer is:
segment
'part' is the definition.
(segment is a kind of part)
'regulate boxing agents' is the wordplay.
'regulate' becomes 'set' (regulate can mean to set standards or rules).
'boxing' means one lot of letters goes inside another ('box' can mean contain, as in to put in a box).
'agents' becomes 'gmen' (G-man is an example).
'set' placed around 'gmen' is 'SEGMENT'.
'in' acts as a link.
(Other definitions for segment that I've seen before include "Part, piece" , "One part of a whole" , "Portion" , "Gets men with part cut off" , "Division" .)