Old king left some of the food offered by butcher? (5)
I believe the answer is:
offal
'some of the food offered by butcher?' is the definition.
(animal entrails)
'old king left' is the wordplay.
'old king' becomes 'offa' (King of Mercia).
'left' becomes 'l' (common abbreviation).
'offa'+'l'='OFFAL'
(Other definitions for offal that I've seen before include "Edible insides" , "Internal organs and entrails as food" , "Worthless parts of animal carcass" , "Internal organs of an animal used as food" , "cheap meat" .)