Prisoner mended structure that was doomed (9)
I believe the answer is:
condemned
'doomed' is the definition.
('condemn' can be a synonym of 'doom')
'prisoner mended structure' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'structure' is an anagram indicator.
'mended' is an anagram of 'demned'.
'con'+'demned'='CONDEMNED'
'that was' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for condemned that I've seen before include "Declared unfit for use" , "Sentenced (to punishment)" , "sent down" , "Censured - pronounced unfit" , "Denounced" .)