One who kills when soldiers do wrong (8)
I believe the answer is:
assassin
'one who kills' is the definition.
'when soldiers do wrong' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'soldiers' becomes 'sas' (Special Air Service).
'do wrong' becomes 'sin' (to sin is to do wrong or act badly).
'as'+'sas'+'sin'='ASSASSIN'
(Other definitions for assassin that I've seen before include "Booth, for example" , "Terrorist" , "Paid killer" , "Killer of famous person" , "A killer of political or religious leader" .)