Aggressive copper, in turn, let off (9)
I believe the answer is:
truculent
'aggressive' is the definition.
('truculent' can be similar in meaning to 'aggressive')
'copper in turn let off' is the wordplay.
I cannot really see how this works, but
'copper' could be 'cu' (chemical symbol) and 'cu' is present in the answer.
'turn' could be 'u' (a 'U-turn' is a kind of turn) and 'u' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for truculent that I've seen before include "Quarrelsome" , "Defiant, rude" , "Rude and aggressive" , "Defiantly aggressive" , "Aggressively defiant" .)