Female's copper, skinny but strong (9)
I believe the answer is:
herculean
'strong' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I don't see?
'female's copper skinny' is the wordplay.
'female' becomes 'her' (I've seen this before).
'copper' becomes 'cu' (chemical symbol).
'skinny' becomes 'lean' (similar in meaning).
'her'+'cu'+'lean'='HERCULEAN'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for herculean that I've seen before include "Showing great strength" , "Powerful" , "for Poirot?" , "Requiring very great effort" , "Monumental (task)" .)