One shoots wading bird and rook (6)
I believe the answer is:
sniper
'one shoots' is the definition.
(type of gunman)
'wading bird and rook' is the wordplay.
'wading bird' becomes 'snipe' (snipe is a kind of wading bird).
'and' is a charade indicator (letters next to each other).
'rook' becomes 'R' (chess abbreviation).
'snipe'+'r'='SNIPER'
(Other definitions for sniper that I've seen before include "One firing from cover" , "He shoots from hiding-place" , "Single marksman in concealed place" , "Person making sly critical remarks, like gunman from hiding?" , "Marksman in hiding place" .)