Meat as a rule with veg is cooked (5,7)
I believe the answer is:
liver sausage
'meat' is the definition.
(liver sausage is a kind of meat)
'as a rule with veg is cooked' is the wordplay.
'with' means one lot of letters go next to another.
'cooked' is an anagram indicator (letters cooked into a new form).
'as'+'a'+'rule'+'veg'+'is'='asarulevegis'
'asarulevegis' is an anagram of 'LIVER SAUSAGE'.
(Other definitions for liver sausage that I've seen before include "deli purchase" , "savoury paste" , "Rolled meat paste" , "savoury spread" .)