Copper said 'Not in disguise as curator (9)
I believe the answer is:
custodian
'curator' is the definition.
(thesaurus)
'copper said 'not in disguise' is the wordplay.
'copper' becomes 'Cu' (Cu is the chemical symbol for copper).
'in disguise' is an anagram indicator.
'said'+'not'='saidnot'
'saidnot' with letters rearranged gives 'stodian'.
'cu'+'stodian'='CUSTODIAN'
'as' is the link.
(Other definitions for custodian that I've seen before include "museum keeper" , "Guardian" , "Protector" , "Protector/guardian (of something)" , "One takes care of" .)