Cancel party after fun fails to start (4)
I believe the answer is:
undo
'cancel' is the definition.
(I've seen this in another clue)
'party after fun fails to start' is the wordplay.
'party' becomes 'do' (do can informally mean a party).
'after' is a charade indicator (letters next to each other).
'fails to start' means to remove the first letter.
'fun' with its first letter taken away is 'un'.
'do' put after 'un' is 'UNDO'.
(Other definitions for undo that I've seen before include "Unfasten or annul" , "Bring to ruin" , "Unravel or ruin" , "Open button or ruin" , "Wreck" .)