Hole in a cooking apple (3)
I believe the answer is:
gap
'hole' is the definition.
(thesaurus)
'in a cooking apple' is the wordplay.
'in' indicates a hidden word.
'GAP' is hidden within 'cooking apple'.
(Other definitions for gap that I've seen before include "Small opening" , "Aperture" , "A break in something continuous, like a hedge" , "Breach or lacuna" , "A small parting, in hedge or teeth" .)