Top copy given wrong mark (4)
I believe the answer is:
apex
'top' is the definition.
(apex is a top or peak)
'copy given wrong mark' is the wordplay.
'copy' becomes 'ape' (ape can mean to copy or imitate).
'given' means one lot of letters go next to another.
'wrong mark' becomes 'x' (X is used to show an incorrect answer).
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "The highest point or pointed end" , "Apogee, summit" , "Absolute peak" , "The tip or highest point (4)" , "climax" .)