Gift for groom having party — and grimacing? (5)
I believe the answer is:
dowry
'gift for groom' is the definition.
(a bride gives a dowry)
'party and grimacing?' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'and' means one lot of letters go next to another.
'grimacing?' becomes 'wry' (wry can mean grimacing or disgusted).
'do'+'wry'='DOWRY'
'having' acts as a link.
(Other definitions for dowry that I've seen before include "Wealth brought by a bride" , "Rowdy (anag.)" , "Bride's property" , "Property bride brings" , "In olden days, the bride's parents paid this to the husband" .)