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