Glutton's first to regret having left porridge (5)
I believe the answer is:
gruel
'porridge' is the definition.
(gruel is a kind of porridge)
'glutton's first to regret having left' is the wordplay.
'first' says to take the initial letters.
'to regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'having' means one lot of letters go next to another.
'left' becomes 'l' (common abbreviation**).
The initial letter of 'gluttons' is 'g'.
'g'+'rue'+'l'='GRUEL'
(Other definitions for gruel that I've seen before include "Thin food of boiled oatmeal" , "Unappetising meal of boiled oatmeal" , "cheap fare" , "Thin oatmeal dish" , "sloppy fare" .)