Core, heart (6)
(Other definitions for centre that I've seen before include "'Things fall apart; the ... cannot hold'/Yeats" , "See 24" , "See 1" , "Point equidistant from all other points" , "Recent (anag)" .)
I believe this clue is a double definition.
'core' is the first definition.
(I've seen this in another clue)
'heart' is the second definition.
(I know that heart can be written as middle)
(Other definitions for middle that I've seen before include "One of 21 [HEARTS]" , "Equally distant from the extremes" , "Group of clubs; distance" , "Median" , "It's equidistant from the extremes" .)