Top copy plus ten (4)
I believe the answer is:
apex
'top' is the definition.
(thesaurus)
'copy plus ten' is the wordplay.
'copy' becomes 'ape' ('ape' can be a synonym of 'copy').
'plus' is a charade indicator (letters next to each other).
'ten' becomes 'x' (roman numeral).
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "Top, highest part" , "Absolute peak" , "Point or summit" , "Tip -- top" , "Acme" .)