Regret going in a heartless gaol for punishment (5)
I believe the answer is:
gruel
'punishment' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'regret going in a heartless gaol' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'going in' indicates putting letters inside.
'a heartless' means to remove the middle letters.
'gaol' with its middle removed is 'gl'.
'rue' going within 'gl' is 'GRUEL'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for gruel that I've seen before include "Luger (anag)" , "Thin liquid food" , "Unappetising food" , "Thin porridge" , "sloppy fare" .)