Affair brings a regret that's curtailed (5)
I believe the answer is:
amour
'affair' is the definition.
(amour can mean a love affair)
'a regret that's curtailed' is the wordplay.
'regret' becomes 'mourn' (I've seen this before).
'that's curtailed' means to remove the last letter.
'mourn' with its last letter taken away is 'mour'.
'a'+'mour'='AMOUR'
'brings' is the link.
(Other definitions for amour that I've seen before include "Love-affair" , "Illicit love" , "Love affair - loved one" , "Romantic affair" , "Loved one (French)" .)