Subvert a foreign party (4)
I believe the answer is:
undo
'subvert' is the definition.
(I've seen this before)
'a foreign party' is the wordplay.
'a foreign' becomes 'un' ('a' translated into French).
'party' becomes 'do' (do is a kind of party).
'un'+'do'='UNDO'
(Other definitions for undo that I've seen before include "Cancel -- annul -- reverse" , "Open or unfasten" , "(4)" , "Cancel - destroy" , "Cause the ruin of, or cancel" .)