Hidden gunman finds bird by river (6)
I believe the answer is:
sniper
'hidden gunman' is the definition.
(I've seen this in another clue)
'bird by river' is the wordplay.
'bird' becomes 'snipe' (snipe is a kind of bird).
'by river' becomes 'r' (abbreviation for river. I am not sure about the 'by' bit.).
'snipe'+'r'='SNIPER'
'finds' acts as a link.
Can you help me to learn more?
(Other definitions for sniper that I've seen before include "Individual firing from cover" , "Concealed shooter" , "Critic" , "Hidden marksman" , "One aiming" .)