Loosen at peacekeepers party (4)
I believe the answer is:
undo
'loosen' is the definition.
(I've seen this before)
'at peacekeepers party' is the wordplay.
'at' is a charade indicator (letters next to each other).
'peacekeepers' becomes 'un'.
'party' becomes 'do' (do is a kind of party).
'un'+'do' is 'UNDO'.
(Other definitions for undo that I've seen before include "Unfasten or annul" , "Ruin or loose" , "Spoil" , "Loosen; cancel" , "Unfasten, loosen" .)