Copper to restrain good-looker (5)
I believe the answer is:
cutie
'good-looker' is the definition.
(I've seen this in another clue)
'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 "Attractive young woman (slang)" , "Dish" , "Endearing person" , "considered by most to be attractive" , "Smart girl" .)