Vain prisoner in the chair, so to speak (9)
I believe the answer is:
conceited
'vain' is the definition.
(synonyms)
'prisoner in the chair so to speak' is the wordplay.
I cannot really see how this works, but
'prisoner' could be 'con' (abbreviation for 'convict') and 'con' is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for conceited that I've seen before include "Very proud of oneself" , "Vain; narcissistic" , "Stuck-up" , "Arrogant" , "With a high opinion of oneself" .)