Earth-shattering organ (5)
I believe the answer is:
'organ' is the definition.
(the heart is an organ of the body)
'earth-shattering' is the wordplay.
'shattering' indicates anagramming the letters.
'earth' with letters rearranged gives 'HEART'.
(Other definitions for heart that I've seen before include "It beats" , "Middle" , "Vital part" , "Decorated (cake)" , "Centre; core" .)