Formal attire which restricts work (3,3)
I believe the answer is:
top hat
'formal attire' is the definition.
(I've seen this before)
'which restricts work' is the wordplay.
'which' becomes 'that' ('that' and 'which' are often synonymous).
'restricts' means one lot of letters goes inside another.
'work' becomes 'op' (short for opus).
'that' going around 'op' is 'TOP-HAT'.
(Other definitions for top hat that I've seen before include "Formal head-wear for a man" , "Formal attire" , "Kind of headgear, goes with 28 down" , "Posh lid" , "Fancy lid" .)