Loosen at a French party (4)
I believe the answer is:
undo
'loosen' is the definition.
(I know that loosen can be written as undo)
'at a french party' is the wordplay.
'at' says to put letters next to each other.
'a french' becomes 'un' ('a' in French).
'party' becomes 'do' (do can informally mean a party).
'un' next to 'do' is 'UNDO'.
(Other definitions for undo that I've seen before include "Cause the ruin of, or cancel" , "Cause to become loose" , "Cause downfall" , "Dismantle - destroy" , "Unfasten, loosen" .)