Loosen at peacekeepers party (4)
I believe the answer is:
Here is my best explanation:
'loosen' is the definition.
(I've seen this before)
'at peacekeepers party' is the subsidiary indication.
'at' means one lot of letters go next to another.
'peacekeepers' becomes 'un'.
'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 "Wreck", "Open buttons or ruin", "Unfasten or annul", "Loosen or reverse", "Bring to ruin", "Ruin or loosen ties", "Loosen; cancel", "Loosen; ruin", "release" and "Cancel -- annul -- reverse".)