Resolute, put off explosive old copper (10)
I believe the answer is:
determined
'resolute' is the definition.
(similar in meaning)
'put off explosive old copper' is the wordplay.
'put off' becomes 'deter' (both can mean to discourage).
'explosive' becomes 'mine' (e.g. a landmine).
'old copper' becomes 'd'.
'deter'+'mine'+'d'='DETERMINED'
(Other definitions for determined that I've seen before include "Decided - resolute" , "Firmly decided" , "firm" , "set" , "Resolved" .)