Sly portion of Cheddar cheese (4)
I believe the answer is:
arch
'sly' is the definition.
(I've seen this in another clue)
'portion of cheddar cheese' is the wordplay.
'portion of' indicates a hidden word.
'ARCH' is hidden in the letters of 'cheddar cheese'.
(Other definitions for arch that I've seen before include "Chief - supporter - cunning" , "Inner side of foot" , "Masonry construction for spanning an opening" , "Chief - opening with curved top" , "Curved part of the foot" .)