Grieve no end after a secret affair (5)
I believe the answer is:
amour
'secret affair' is the definition.
(I've seen this before)
'grieve no end after a' is the wordplay.
'grieve' becomes 'mourn' (mourning is a kind of grieving).
'no end' means to remove the last letter.
'after' is a charade indicator (letters next to each other).
'mourn' with its final letter taken off is 'mour'.
'mour' after 'a' is 'AMOUR'.
(Other definitions for amour that I've seen before include "Fling" , "Secretive affair" , "close relationship" , "(Secret) lover" , "Affair of the heart" .)