Outlet for soldier’s natural aggression, as fighting is, primarily (5)
I believe the answer is:
naafi
'outlet for soldier's' is the definition.
(Navy, Army and Air Force Institutes store)
'natural aggression as fighting is primarily' is the wordplay.
'primarily' says to take the initial letters.
The first letters of 'natural aggression as fighting is' is 'NAAFI'.
(Other definitions for naafi that I've seen before include "Forces 17" , "military institution" , "eatery" , "mess" , "Military facility" .)