Mike's wearing great top (4)
I believe the answer is:
acme
'top' is the definition.
(synonyms)
'mike's wearing great' is the wordplay.
'mike' becomes 'm' (phonetic alphabet: alpha, bravo, charlie etc.).
'wearing' is an insertion indicator.
'great' becomes 'ace' (I've seen this before).
'm' inserted into 'ace' is 'ACME'.
(Other definitions for acme that I've seen before include "Highest level or degree attainable" , "Culmination" , "Summit - maximum" , "Height of perfection" , "Top or highest point" .)