The temperature right in the middle (5)
I believe the answer is:
heart
'the middle' is the definition.
('heart' can be a synonym of 'middle')
'the temperature right in' is the wordplay.
'the temperature' becomes 'heat' (heat is a kind of temperature).
'right' becomes 'r' (common abbreviation).
'in' indicates putting letters inside.
'heat' going around 'r' is 'HEART'.
(Other definitions for heart that I've seen before include "Earth (anag) -- compassion" , "Blood-pumping organ" , "Not a club" , "Core; organ" , "Player for Scottish team" .)