Twist decree by ambassador (6)
I believe the answer is:
writhe
'twist' is the definition.
(synonyms)
'decree by ambassador' is the wordplay.
'decree' becomes 'writ' (I've seen this in another clue).
'by' says to put letters next to each other.
'ambassador' becomes 'HE' (His or Her Excellency).
'writ'+'he'='WRITHE'
(Other definitions for writhe that I've seen before include "Painfully move" , "Contort in pain" , "6, maybe in agony" , "Squirm in pain" , "'Twist, squirm (6)'" .)