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