Porridge in heartless gaol, full of regret (5)
I believe the answer is:
gruel
'porridge' is the definition.
(gruel is a kind of porridge)
'heartless gaol full of regret' is the wordplay.
'heartless' means to remove the middle letters.
'full of' is an insertion indicator.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'gaol' with its centre taken out is 'gl'.
'gl' enclosing 'rue' is 'GRUEL'.
'in' acts as a link.
(Other definitions for gruel that I've seen before include "disappointing fare" , "cheap fare" , "Thin porridge" , "Thin oatmeal dish" , "Unappetising meal of boiled oatmeal" .)