Copper to restrain attractive girl (5)
I believe the answer is:
cutie
'attractive girl' is the definition.
(I've seen this before)
'copper to restrain' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'to restrain' becomes 'tie' (tying is a kind of restraining).
'cu'+'tie'='CUTIE'
(Other definitions for cutie that I've seen before include "lovely" , "an attractive thing" , "Attractive girl" , "Smasher" , "Smart girl" .)