Prisoner kind to companion (7)
I believe the answer is:
consort
'companion' is the definition.
(thesaurus)
'prisoner kind' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'kind' becomes 'sort' ('sort' can be a synonym of 'kind'**).
'con'+'sort'='CONSORT'
'to' acts as a link.
(Other definitions for consort that I've seen before include "Husband or wife of a monarch" , "Marriage partner" , "Spouse, partner" , "helpmate" , "A monarch's spouse" .)