Imagine groom's costume (5,5)
I believe the answer is:
fancy dress
'costume' is the definition.
(fancy dress is a kind of costume)
'imagine groom's' is the wordplay.
'imagine' becomes 'fancy' (fancying is a kind of imagining).
'groom' becomes 'dress' ('dress' can be a synonym of 'groom').
'fancy'+'dress'='FANCY DRESS'
(Other definitions for fancy dress that I've seen before include "Themed party attire" , "Masquerade costume?" , "Frivolous party garb" , "Party costume" .)