Principal ingredient of Cheddar cheese (4)
I believe the answer is:
arch
'principal' is the definition.
(as in arch-enemy)
'ingredient of cheddar cheese' is the wordplay.
'ingredient of' indicates a hidden word.
'ARCH' can be found hidden inside 'cheddar cheese'.
(Other definitions for arch that I've seen before include "Char the curved structure" , "Embow" , "Supporting structure" , "in short, is out of date" , "Curved vault" .)