Essence of fruit stone hearts (4)
I believe the answer is:
pith
'essence' is the definition.
('pith' can be a synonym of 'essence')
'fruit stone hearts' is the wordplay.
'fruit stone' becomes 'pit' (I've seen this before).
'hearts' becomes 'h' (abbreviation used in card games e.g. bridge).
'pit'+'h'='PITH'
'of' is the link.
(Other definitions for pith that I've seen before include "hats may be made of this" , "Fibrous part of fruit - essence" , "Soft tissue; essence" , "Spongy tissue like that lining an orange skin" , "Essence; tissue inside rind" .)