Of the heart (7)
I believe the answer is:
cardiac
'of the heart' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cardiac that I've seen before include "Appertaining to the heart" , "Sort of surgery" , "Pertaining to the heart" , "Relating to the heart" , "Relating to 8 down, part one" .)