Heating unit and the room (5)
I believe the answer is:
therm
'heating unit' is the definition.
(I've seen this in another clue)
'the room' is the wordplay.
'room' becomes 'rm' (abbreviation).
'the'+'rm'='THERM'
'and' acts as a link.
(Other definitions for therm that I've seen before include "A hot unit" , "Unit of measurement of heat" , "Former gas supply unit" , "Unit of heat as in gas supply" , "Gasman's unit" .)