Set criminal prisoner back around middle of stretch (7)
I believe the answer is:
congeal
'set' is the definition.
('congeal' can be a synonym of 'set')
'criminal prisoner back around middle of stretch' is the wordplay.
'criminal' becomes 'con' (abbreviation for 'convict').
'prisoner back' becomes 'gal' (I can't explain this - if you can you should give a lot more credence to this answer).
'around' indicates putting letters inside.
'middle of' says to take the centre.
The middle letter of 'stretch' is 'e'.
'gal' enclosing 'e' is 'geal'.
'con'+'geal'='CONGEAL'
Can you help me to learn more?
(Other definitions for congeal that I've seen before include "Become gelatinous" , "Solidify by cooling" , "Coagulate" , "Become semi-solid like cooling grease" , "cake" .)