Cleric to evaluate copper first (6)
I believe the answer is:
curate
'cleric' is the definition.
(I've seen this in another clue)
'evaluate copper first' is the wordplay.
'evaluate' becomes 'rate' (rating is a kind of evaluating).
'copper' becomes 'Cu' (chemical symbol).
'first' means one lot of letters go next to another.
'rate' put after 'cu' is 'CURATE'.
'to' is the link.
(Other definitions for curate that I've seen before include "Clergyman assisting a vicar" , "He didn't really want an egg" , "Junior church minister" , "He whose egg is good in parts" , "assistant rector" .)