One who shoots bird by river (6)
I believe the answer is:
sniper
'one who shoots' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition. I also know that 'shoots' relates to this answer.
'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'
Can you help me to learn more?
(Other definitions for sniper that I've seen before include "Person making sly critical remarks, like gunman from hiding?" , "He shoots from hiding-place" , "Concealed gunman" , "He fires shots from hidingplace" , "Rifleman firing from concealed location" .)