Agree Tory must leave, being scoundrel (3)
I believe the answer is:
cur
'being scoundrel' is the definition.
(cur can mean a despicable person)
'agree tory must leave' is the wordplay.
'agree' becomes 'concur' (synonyms).
'tory' becomes 'con' (abbreviation for Conservative).
'must leave' is a deletion indicator.
'concur' with 'con' taken out is 'CUR'.
(Other definitions for cur that I've seen before include "Aggressive dog" , "Wretch" , "Useless dog" , "Worthless or bad-tempered dog" , "Non-pedigree dog" .)