Marksman found wading bird with rook (6)
I believe the answer is:
sniper
'marksman' is the definition.
(sniper is a kind of marksman)
'wading bird with rook' is the wordplay.
'wading bird' becomes 'snipe' (snipe is a kind of wading bird).
'with' is a charade indicator (letters next to each other).
'rook' becomes 'r' (abbreviation in chess).
'snipe'+'r'='SNIPER'
'found' acts as a link.
(Other definitions for sniper that I've seen before include "Single marksman in concealed place" , "Marksman in hiding place" , "Concealed gunman" , "one who takes pot from a safe place" , "Hidden marksman" .)