Prisoner, terribly large to me, is a mixture (12)
I believe the answer is:
conglomerate
'mixture' is the definition.
(thesaurus)
'prisoner terribly large to me' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'terribly' indicates an anagram.
'large'+'to'+'me'='largetome'
'largetome' is an anagram of 'glomerate'.
'con'+'glomerate'='CONGLOMERATE'
'is a' is the link.
(Other definitions for conglomerate that I've seen before include "Large corporation formed through mergers" , "Multinational" , "Big diverse firm" , "Coarse-grained rock; large corporation" , "Large company formed by merger" .)