Kind prisoner starts to agree (7)
I believe the answer is:
consort
'agree' is the definition.
(I know that agree can be written as consort)
'kind prisoner starts' is the wordplay.
'kind' becomes 'sort' (synonyms).
'prisoner' becomes 'con' (abbreviation for 'convict').
'starts' is a charade indicator (letters next to each other).
'sort' put after 'con' is 'CONSORT'.
'to' is the link.
(Other definitions for consort that I've seen before include "helpmate" , "Spouse, partner" , "Prince Albert?" , "Marriage partner" , "Small group; spouse" .)