One shooting bird on river (6)
I believe the answer is:
sniper
'one shooting' is the definition.
(person who shoots a sniper gun)
'bird on river' is the wordplay.
I cannot really understand how this works, but
'bird' could be 'snipe' (snipe is a kind of bird) and 'snipe' is found within the answer.
'river' could be 'r' (abbreviation for river) and 'r' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sniper that I've seen before include "Concealed sharpshooter" , "He shoots from hiding-place" , "Good shot" , "One constantly criticising" , "one who takes pot from a safe place" .)