Grieve endlessly following a romantic affair (5)
I believe the answer is:
amour
'romantic affair' is the definition.
(I've seen this in another clue)
'grieve endlessly following a' is the wordplay.
'grieve' becomes 'mourn' (mourning is a kind of grieving).
'endlessly' means to remove the last letter.
'following' says to put letters next to each other.
'mourn' with its final letter taken away is 'mour'.
'mour' put after 'a' is 'AMOUR'.
(Other definitions for amour that I've seen before include "French romance" , "illicit liaison" , "Loved one (French)" , "Sexual liaison" , "Illicit love" .)