Heart, centre (4)
I believe the answer is:
core
This clue is a double definition.
'heart' is the first definition.
(I know that heart can be written as core)
'centre' is the second definition.
(core can mean something's centre)
(Other definitions for core that I've seen before include "Nucleus" , "Crux" , "essential element" , "Central part of the Earth" , "Planet's centre" .)