Agree to deceive a dirty dog! (6)
I believe the answer is:
concur
'agree' is the definition.
('concur' can be a synonym of 'agree')
'deceive a dirty dog' is the wordplay.
'deceive' becomes 'con' (I've seen this before).
'a dirty dog' becomes 'cur'.
'con'+'cur'='CONCUR'
'to' is the link.
(Other definitions for concur that I've seen before include "'Agree, coincide (6)'" , "Agree or happen together" , "share the same opinion" , "run together" , "See eye to eye" .)