Loosen in 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'
'in' acts as a link.
(Other definitions for undo that I've seen before include "Cause to become loose" , "set free" , "Unfasten - ruin" , "Take back" , "Untie or ruin" .)