Crown featuring mock cross (4)
I believe the answer is:
apex
'crown' is the definition.
(I've seen this before)
'mock cross' is the wordplay.
'mock' becomes 'ape' (aping is a kind of mocking).
'cross' becomes 'x' (an X is a cross).
'ape'+'x'='APEX'
'featuring' acts as a link.
(Other definitions for apex that I've seen before include "Top, highest part" , "Top, peak" , "Uppermost point" , "Peak, vertex" , "Point or summit" .)