Copper needs right dog (3)
I believe the answer is:
cur
'dog' is the definition.
(both can mean a despicable person)
'copper needs right' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'needs' says to put letters next to each other.
'right' becomes 'r' (common abbreviation).
'cu'+'r'='CUR'
(Other definitions for cur that I've seen before include "Mongrel - cowardly person" , "Vicious dog" , "Vile dog" , "Worthless or bad-tempered dog" , "Wretch" .)