Work in that formal wear (3,3)
I believe the answer is:
top hat
'formal wear' is the definition.
(I've seen this before)
'work in that' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'in' indicates putting letters inside.
'op' inserted into 'that' is 'TOP-HAT'.
(Other definitions for top hat that I've seen before include "Kind of headgear, goes with 28 down" , "Formal head-wear for a man" , "Wear it with tails" , "Formal headgear for man" , "Film starring Fred Astaire" .)