Endless regret after a love affair (5)
I believe the answer is:
amour
'love affair' is the definition.
(I've seen this before)
'endless regret after a' is the wordplay.
'endless' means to remove the last letter.
'regret' becomes 'mourn' (I've seen this before).
'after' means one lot of letters go next to another.
'mourn' with its final letter taken away is 'mour'.
'mour' after 'a' is 'AMOUR'.
(Other definitions for amour that I've seen before include "Illicit love" , "Love affair - loved one" , "French love affair, usually secret" , "(Illicit) sexual relationship" , "Result of Cupid's darts" .)