Twist in legal document supported by ambassador (6)
I believe the answer is:
writhe
'twist' is the definition.
('writhe' can be a synonym of 'twist')
'legal document supported by ambassador' is the wordplay.
'legal document' becomes 'writ' (writ is a kind of legal document).
'supported by' means one lot of letters go next to another (letters go below - in down clue).
'ambassador' becomes 'he' (His or Her Excellency).
'writ'+'he'='WRITHE'
'in' is the link.
(Other definitions for writhe that I've seen before include "Twist as in pain" , "Move sinuously" , "Thrash" , "Struggle" , "Painfully move" .)