Prisoner posted agreement (7)
I believe the answer is:
consent
'agreement' is the definition.
(consent is a kind of agreement)
'prisoner posted' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'posted' becomes 'sent' ('send' can be a synonym of 'post').
'con'+'sent'='CONSENT'
(Other definitions for consent that I've seen before include "Agree (to do something)" , "authorisation" , "Permission, agree" , "Say yes to something" , "Agree or comply" .)