Prisoners, kind to get locked in as mates (8)
I believe the answer is:
consorts
'mates' is the definition.
('consort' can be a synonym of 'mate')
'prisoners kind to get locked in' is the wordplay.
'prisoners' becomes 'cons' (short for convicts).
'kind' becomes 'sort' (synonyms).
'to get locked in' is an insertion indicator.
'cons' enclosing 'sort' is 'CONSORTS'.
'as' acts as a link.
(Other definitions for consorts that I've seen before include "Partners, companions" , "Hangs out with" , "Scorns to mix with royal couples" , "Scorns to mix with royal husbands or wives" .)