Three in 651 get nasty (9)
I believe the answer is:
vitriolic
'nasty' is the definition.
Both the definition and answer are adjectives. Maybe there's an association between them I don't understand?
'three in 651' is the wordplay.
I cannot quite understand how this works, but
'three' could be 'trio' (synonyms) and 'trio' is found in the answer.
This explanation may well be incorrect...
'get' is the link.
Can you help me to learn more?
(Other definitions for vitriolic that I've seen before include "Highly corrosive" , "Bitterly critical" , "biting" , "Acrimonious" , "Severely bitter" .)