Top copy that's wrong (4)
I believe the answer is:
apex
'top' is the definition.
(apex is a top or peak)
'copy that's wrong' is the wordplay.
'copy' becomes 'ape' (ape can mean to copy or imitate).
'that's wrong' becomes 'x' (X is used to mark something as incorrect).
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "Uppermost tip" , "Peak, vertex" , "Culmination" , "Topmost point" , "The highest point of something" .)