Principal groomsman at a wedding (4,3)
I believe the answer is:
best man
'principal groomsman at a wedding' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for best man that I've seen before include "Supporting male actor at a wedding" , "Important supporting role at a wedding" , "The groom's right-hand supporter" , "Key union facilitator r" , "Bridegroom's mate" .)