Many a PX shopper (3)
I believe the answer is:
nco
'many a px shopper' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nco that I've seen before include "eg RSM" , "In brief, a corporal or sergeant" , "lancejack, say" , "Initially, a corporal or sergeant" , "eg Corporal" .)