Hardened inmate is discontented, creating a difficult problem (7)
I believe the answer is:
toughie
'a difficult problem' is the definition.
(I've seen this before)
'hardened inmate is discontented' is the wordplay.
'hardened' becomes 'tough' (I can't justify this - if you can you should believe this answer much more).
'is discontented' means to remove the middle letters (the word's contents are removed).
'inmate' with its middle taken out is 'ie'.
'tough'+'ie'='TOUGHIE'
'creating' is the link.
Can you help me to learn more?
(Other definitions for toughie that I've seen before include "It may prove impossible" , "Difficult problem" , "poser" , "Yob" , "A difficult test" .)