Costume for groom? (5)
I believe the answer is:
dress
I believe this is a double definition.
'costume' is the first definition.
(I've seen this in other clues)
'groom?' is the second definition.
(dressing is a kind of grooming)
'for' is the link.
(Other definitions for dress that I've seen before include "Clothe; decorate" , "Apparel" , "Straighten the line" , "Costume" , "One-piece garment" .)