Zealot in cooler upstairs room losing heart (7)
I believe the answer is:
fanatic
'zealot' is the definition.
(I've seen this before)
'cooler upstairs room losing heart' is the wordplay.
'cooler' becomes 'fan' (something which cools).
'upstairs room' becomes 'attic' (I've seen this before).
'losing heart' suggests removing the centre.
'attic' with its middle removed is 'atic'.
'fan'+'atic'='FANATIC'
'in' acts as a link.
(Other definitions for fanatic that I've seen before include "Over-enthusiastic person" , "Extreme zealot" , "enthusiastic" , "crank" , "Bigot - enthusiast" .)