Centre of earth in high temperature core (5)
I believe the answer is:
heart
'core' is the definition.
('heart' can be a synonym of 'core')
'centre of earth in high temperature' is the wordplay.
'centre' says to take the centre.
'of earth' becomes 'terrene' (I've seen this before).
'in' means one lot of letters goes inside another.
'high temperature' becomes 'heat' ('heat' can be a synonym of 'high temperature').
The middle of 'terrene' is 'r'.
'r' put inside 'heat' is 'HEART'.
(Other definitions for heart that I've seen before include "Core, courage" , "It's in the chest" , "Player for Scottish team" , "Not a club" , "the name of a prison in Midlothian!" .)