Ruin a Parisian party (4)
I believe the answer is:
undo
'ruin' is the definition.
(someone's undoing is their ruin)
'a parisian party' is the wordplay.
'a parisian' becomes 'un' ('a' in French).
'party' becomes 'do' (do can informally mean a party).
'un'+'do'='UNDO'
(Other definitions for undo that I've seen before include "Cause the ruin of, or cancel" , "Ruin or loosen ties" , "Bring to nought" , "Take apart" , "Loosen; annul" .)