Top copy going to Number Ten (4)
I believe the answer is:
apex
'top' is the definition.
(apex is a top or peak)
'copy going to number ten' is the wordplay.
'copy' becomes 'ape' (ape can mean to copy or imitate).
'going to' means one lot of letters go next to another.
'number ten' becomes 'x' (Roman numeral).
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "Absolute peak" , "Tip -- top" , "Acme" , "Summit - pinnacle" , "crown" .)