Main, one fed to cook (5)
I believe the answer is:
chief
'main' is the definition.
(I've seen this before)
'one fed to cook' is the wordplay.
'one' becomes 'i' (Roman numeral).
'fed to' is an insertion indicator.
'cook' becomes 'chef' (chef is a kind of cook**).
'i' placed inside 'chef' is 'CHIEF'.
(Other definitions for chief that I've seen before include "Person in charge, head" , "Gaffer" , "Tribal head" , "Organisation head" , "Tribal leader" .)