Heart that's hollow and hard (4)
I believe the answer is:
pith
'heart' is the definition.
('pith' can be a synonym of 'heart')
'hollow and hard' is the wordplay.
'hollow' becomes 'pit' (pit is a kind of hollow).
'and' is a charade indicator (letters next to each other).
'hard' becomes 'h'.
'pit'+'h'='PITH'
'that's' acts as a link.
(Other definitions for pith that I've seen before include "Medulla" , "Essence; tissue inside rind" , "Essential substance" , "Citrus rind lining" , "Inside skin of, say, orange" .)