Regret being in empty gaol for porridge (5)
I believe the answer is:
gruel
'porridge' is the definition.
(gruel is a kind of porridge)
'regret being in empty gaol' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'being in' means one lot of letters goes inside another.
'empty' says to hollow out the word (remove centre letters).
'gaol' with its middle removed is 'gl'.
'rue' put inside 'gl' is 'GRUEL'.
'for' acts as a link.
(Other definitions for gruel that I've seen before include "Unappetising meal of boiled oatmeal" , "Thin liquid food" , "cheap fare" , "Thin soup" , "Luger (anag)" .)