Couple beginning to regret row (4)
I believe the answer is:
tier
'row' is the definition.
(a tier is a row of things)
'couple beginning to regret' is the wordplay.
'couple' becomes 'tie' (both can mean to join).
'beginning to' suggests taking the first letters.
The initial letter of 'regret' is 'r'.
'tie'+'r'='TIER'
(Other definitions for tier that I've seen before include "Rite (anag.)" , "Plague" , "Person joining" , "Rite about layer" , "he hasn't won?" .)