Rat caught by copper or keeper (7)
I believe the answer is:
curator
'keeper' is the definition.
(I know that curator is a type of keeper)
'rat caught by copper or' is the wordplay.
'caught by' is an insertion indicator.
'copper' becomes 'cu' (chemical symbol).
'cu'+'or'='cuor'
'rat' inserted within 'cuor' is 'CURATOR'.
(Other definitions for curator that I've seen before include "Guardian" , "man in the museum?" , "Keeper of a collection" , "Custodian of a collection" , "Museum official" .)