Killer shows when to commit offence with donkey (8)
I believe the answer is:
assassin
'killer' is the definition.
(I know that killer can be written as assassin)
'when to commit offence with donkey' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'to commit offence' becomes 'sin' (I can't explain this - if you can you should give a lot more credence to this answer).
'with' says to put letters next to each other.
'donkey' becomes 'ass' (ass is a term for a donkey).
'as'+'sin'='assin'
'assin' after 'ass' is 'ASSASSIN'.
'shows' acts as a link.
Can you help me to learn more?
(Other definitions for assassin that I've seen before include "One making hits" , "Political/religious killer" , "hit man" , "Professional, often political, killer" , "Hired murderer" .)