Groom's attendant (4,3)
I believe the answer is:
best man
'groom's attendant' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for best man that I've seen before include "The groom's right-hand supporter" , "Bridegroom's friend" , "Bloke participating in match" , "Key union facilitator r" , "Bridegroom's mate" .)