Prisoner and scoundrel agree (6)
I believe the answer is:
concur
'agree' is the definition.
('concur' can be a synonym of 'agree')
'prisoner and scoundrel' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'and' says to put letters next to each other.
'scoundrel' becomes 'cur' (I've seen this before).
'con'+'cur'='CONCUR'
(Other definitions for concur that I've seen before include "Stone pile" , "meet" , "Give assent" , "Agree or happen together" , "Be in accord" .)