Top mimic getting cross (4)
I believe the answer is:
apex
'top' is the definition.
(apex is a top or peak)
'mimic getting cross' is the wordplay.
'mimic' becomes 'ape' (ape can mean to copy or imitate).
'getting' says to put letters next to each other.
'cross' becomes 'x' (an X is a cross).
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "The highest point or peak" , "Summit - pinnacle" , "Cheap fare" , "crown" , "high-point" .)