Is able to catch disheartened inmates with prohibited substance (8)
I believe the answer is:
cannabis
'prohibited substance' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'is able to catch disheartened inmates' is the wordplay.
'is able to' becomes 'can' (if you can do something you are able to do it).
'catch' becomes 'nab' (nab can mean to capture or arrest).
'disheartened' suggests removing the centre (the middle or heart is removed).
'inmates' with its centre taken out is 'is'.
'can'+'nab'+'is'='CANNABIS'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for cannabis that I've seen before include "Most commonly used (mostly) illicit drug" , "Ban Incas from using this drug" , "Marijuana" , "illegal drug supply" , "Strangely, Incas ban this drug" .)