Opening portion of macaroni cheese (5)
I believe the answer is:
niche
'opening' is the definition.
'portion of macaroni cheese' is the wordplay.
'portion of' indicates a hidden word.
'NICHE' is hidden within 'macaroni cheese'.
(Other definitions for niche that I've seen before include "Place that's cosy" , "Suitable position or small alcove" , "Speciality" , "small specialised market" , "Special location" .)