Stone with hard core (4)
I believe the answer is:
pith
'core' is the definition.
(I've seen this before)
'stone with hard' is the wordplay.
'stone' becomes 'pit' ('pit' can be a synonym of 'stone').
'with' says to put letters next to each other.
'hard' becomes 'h'.
'pit'+'h'='PITH'
(Other definitions for pith that I've seen before include "Lining of rind; essence" , "Core - essence" , "Marrow (of plant)" , "Spongy tissue lining rinds of oranges, or the essential part" , "hats may be made of this" .)