Accustomed to employ copper (4)
I believe the answer is:
used
'accustomed' is the definition.
(thesaurus)
'employ copper' is the wordplay.
'employ' becomes 'use' ('use' can be a synonym of 'employ').
'copper' becomes 'd'.
'use'+'d'='USED'
'to' acts as a link.
(Other definitions for used that I've seen before include "Subject to exploitation" , "Brought into service" , "like majority of cars?" , "Employed in task, or second-hand" , "Exploited - not new" .)