Prisoner, solitary, had finally to be comforted (8)
I believe the answer is:
consoled
'be comforted' is the definition.
(console can mean to comfort or sympathise with)
'prisoner solitary had finally' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'solitary' becomes 'sole' (synonyms).
'finally' suggests the final letters.
The final letter of 'had' is 'd'.
'con'+'sole'+'d'='CONSOLED'
'to' acts as a link.
(Other definitions for consoled that I've seen before include "Comforted in time of sorrow" , "Gave comfort to (someone)" , "Cheered" .)