Agree to swindle scoundrel (6)
I believe the answer is:
concur
'agree' is the definition.
(I know that agree can be written as concur)
'swindle scoundrel' is the wordplay.
'swindle' becomes 'con' (synonyms).
'scoundrel' becomes 'cur' (cur can mean a despicable person).
'con'+'cur'='CONCUR'
'to' is the link.
(Other definitions for concur that I've seen before include "Be in accord" , "Express agreement" , "meet" , "share the same opinion" , "See eye to eye" .)