Copper to marry a pretty girl (5)
I believe the answer is:
cutie
'a pretty girl' is the definition.
(I've seen this in another clue)
'copper to marry' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'to marry' becomes 'tie' (synonyms).
'cu'+'tie'='CUTIE'
(Other definitions for cutie that I've seen before include "Smart girl" , "Smart female" , "something pleasing" , "Appealing young lady" , "lovely" .)