Gunman again pegs back (6)
I believe the answer is:
sniper
'gunman' is the definition.
(sniper is a kind of gunman)
'again pegs back' is the wordplay.
'again' becomes 're'.
'pegs' becomes 'pins' ('pin' can be a synonym of 'peg').
'back' says the letters should be written in reverse.
're'+'pins'='repins'
'repins' back-to-front is 'SNIPER'.
(Other definitions for sniper that I've seen before include "frets about having lost sixth place" , "Good shot" , "Military marksman" , "Hidden sharpshooter" , "One shoots from a concealed place" .)