One kills when best troops do wrong (8)
I believe the answer is:
assassin
'one kills' is the definition.
'when best troops do wrong' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'best troops' becomes 'sas' (special air service. I am not sure about the 'best' bit.).
'do wrong' becomes 'sin' (to sin is to do wrong or act badly).
'as'+'sas'+'sin'='ASSASSIN'
Can you help me to learn more?
(Other definitions for assassin that I've seen before include "Booth, for example" , "Murderer, killer" , "Political murderer" , "Murderer (of prominent figure)" , "I kill" .)