Top copy letter (4)
I believe the answer is:
apex
'top' is the definition.
(apex is a top or peak)
'copy letter' is the wordplay.
'copy' becomes 'ape' (synonyms).
'letter' becomes 'x' ().
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "white collar union" , "Uppermost tip" , "The highest point of something" , "Topmost point" , "climax" .)