Core of apple hit different core (4)
I believe the answer is:
'core' is the definition.
(I know that core can be written as pith)
'core of apple hit different' is the wordplay.
'core of' indicates the central letters.
'different' is an anagram indicator (letters placed in a different order).
The middle letter of 'apple' is 'p'.
'hit' is an anagram of 'ith'.
(Other definitions for pith that I've seen before include "Essence; tissue inside rind" , "Lining of rind; essence" , "Tissue inside the rind of citrus fruits" , "hats may be made of this" , "Spongy tissue lining rinds of oranges, or the essential part" .)