One criticising marksman? (6)
I believe the answer is:
sniper
This clue is a double definition.
'one criticising' is the first definition.
(someone who snipes)
'marksman?' is the second definition.
(I know that marksman can be written as sniper)
(Other definitions for sniper that I've seen before include "Rifleman firing from concealed location" , "He shoots from a hidden position" , "One aiming" , "Concealed gunman" , "Marksman in hiding place" .)