Loosen a French party (4)
I believe the answer is:
undo
'loosen' is the definition.
(I know that loosen can be written as undo)
'a french party' is the wordplay.
'a french' becomes 'un' ('a' in French).
'party' becomes 'do' (do is a kind of party).
'un'+'do'='UNDO'
(Other definitions for undo that I've seen before include "Loosen (knot)" , "Bring to nought" , "Free from ties, laces, etc" , "set free" , "Reverse effect of" .)