Top copy, ten required (4)
I believe the answer is:
apex
'top' is the definition.
(thesaurus)
'copy ten required' is the wordplay.
'copy' becomes 'ape' ('ape' can be a synonym of 'copy').
'ten' becomes 'x' (roman numeral).
'required' says to put letters next to each other.
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "Uppermost tip" , "Uppermost point" , "Acme" , "Tip -- top" , "Summit - pinnacle" .)