Top mimic has that unknown factor (4)
I believe the answer is:
apex
'top' is the definition.
(apex is a top or peak)
'mimic has that unknown factor' is the wordplay.
'mimic' becomes 'ape' (ape can mean to copy or imitate).
'has' is a charade indicator (letters next to each other).
'that unknown factor' becomes 'x' (algebra).
'ape'+'x'='APEX'
(Other definitions for apex that I've seen before include "Culmination" , "The highest point of something" , "Top, highest part" , "Summit - pinnacle" , "white collar union" .)