Cause the downfall of United Nations party (4)
I believe the answer is:
undo
'cause the downfall of' is the definition.
(I've seen this in another clue)
'united nations party' is the wordplay.
'united nations' becomes 'UN' (abbreviation).
'party' becomes 'do' (do is a kind of party).
'un'+'do'='UNDO'
(Other definitions for undo that I've seen before include "Loosen; cancel" , "Cause downfall" , "set free" , "Unravel or ruin" , "Open buttons or ruin" .)