Force prisoners to develop skills (9)
I believe the answer is:
constrain
'force' is the definition.
(I've seen this before)
'prisoners to develop skills' is the wordplay.
'prisoners' becomes 'cons' (short for convicts).
'to develop skills' becomes 'train' (I can't justify this - if you can you should give a lot more credence to this answer).
'cons'+'train'='CONSTRAIN'
Can you help me to learn more?
(Other definitions for constrain that I've seen before include "Compel towards a course of action" , "Lock up" , "Check" , "Confine, limit" , "Force" .)