Reps in trouble with gunman (6)
I believe the answer is:
sniper
'gunman' is the definition.
(sniper is a kind of gunman)
'reps in trouble' is the wordplay.
'trouble' is an anagram indicator.
'reps'+'in'='repsin'
'repsin' with letters rearranged gives 'SNIPER'.
'with' acts as a link.
(Other definitions for sniper that I've seen before include "One shooting from concealed position" , "Individual firing from cover" , "Person making sly critical remarks, like gunman from hiding?" , "One who criticises" , "Lone rifleman" .)