Loosen at United Nations party (4)
I believe the answer is:
undo
'loosen' is the definition.
(I know that loosen can be written as undo)
'at united nations party' is the wordplay.
'at' is a charade indicator (letters next to each other).
'united nations' becomes 'UN' (abbreviation).
'party' becomes 'do' (do is a kind of party).
'un' put next to 'do' is 'UNDO'.
(Other definitions for undo that I've seen before include "set free" , "Reverse effect of" , "Untie or ruin" , "Ruin or loosen ties" , "Release fastening" .)