Relating to the heart (7)
I believe the answer is:
cardiac
'relating to the heart' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cardiac that I've seen before include "Sort of surgery" , "Pertaining to the heart" , "Relating to 8 down, part one" , "Of the heat, perhaps illness" , "of hearts" .)