Agree to put a hundred on the dog (6)
I believe the answer is:
concur
'agree' is the definition.
(I know that agree can be written as concur)
'a hundred on the dog' is the wordplay.
'a hundred' becomes 'C' (Roman numeral).
'the dog' becomes 'cur' (both can mean a despicable person).
'c'+'on'+'cur'='CONCUR'
'to put' is the link.
(Other definitions for concur that I've seen before include "share the same opinion" , "Be in accord" , "run together" , "Agree or happen together" , "'Agree, coincide (6)'" .)