Next formal wear's excellent, worker admits (8)
I believe the answer is:
adjacent
'next' is the definition.
(I've seen this before)
'formal wear's excellent worker admits' is the wordplay.
'formal wear' becomes 'dj' (abbreviation for dinner jacket).
'excellent' becomes 'ace' (I've seen this before**).
'worker' becomes 'ant' (worker is a type of ant).
'admits' is an insertion indicator.
'dj'+'ace'='djace'
'djace' going within 'ant' is 'ADJACENT'.
(Other definitions for adjacent that I've seen before include "next door" , "Contiguous" , "Alongside" , "cheek by jowl" , "Neither opposite nor hypotenuse?" .)