Has to support grand formal wear (5)
I believe the answer is:
gowns
'formal wear' is the definition.
'has to support grand' is the wordplay.
'has' becomes 'owns' ('own' can be a synonym of 'have').
'to support' is a charade indicator (letters next to each other).
'grand' becomes 'g' ('G' can be a synonym of 'grand').
'owns' put after 'g' is 'GOWNS'.
(Other definitions for gowns that I've seen before include "academic gear" , "Fancy frocks" , "Long flowing dresses for women" , "formal attire" , "Theatre outfits" .)