Loosen at a French party (4)
I believe the answer is:
Here is my best explanation:
'loosen' is the definition.
(I've seen this before)
'at a french party' is the subsidiary indication.
'at' says to put letters next to each other.
'a french' becomes 'un' ('a' transated into the French language).
'party' becomes 'do' (do is a kind of party).
'un'+'do' is 'undo'.
(Other definitions for undo that I've seen before include "Open buttons or ruin", "Unfasten - destroy", "Disentangle", "Nullify", "release", "Loose", "Cause downfall", "Loosen (ties)", "Loosen (knot)" and "Subvert".)