Dress in unfashionable suit (6)
I believe the answer is:
outfit
'dress' is the definition.
(outfit is a kind of dress)
'unfashionable suit' is the wordplay.
'unfashionable' becomes 'out' ('out' can be a synonym of 'unfashionable').
'suit' becomes 'fit' ('fit' can be a synonym of 'suit'**).
'out'+'fit'='OUTFIT'
'in' is the link.
(Other definitions for outfit that I've seen before include "Kit, rig-out" , "Complete set of clothes" , "Business" , "what to wear?" , "suit" .)