Artist starts to draw in a tiny, overcrowded room - it heats up (8)
I believe the answer is:
radiator
'it heats up' is the definition.
'artist starts to draw in a tiny' is the wordplay.
'artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'starts to' says to take the initial letters.
'tiny' becomes 'tor' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The initial letters of 'draw in' is 'di'.
'ra'+'di'+'a'+'tor'='RADIATOR'
'overcrowded room' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
(Other definitions for radiator that I've seen before include "House-warmer; engine-cooler" , "It spreads heat as in house" , "Aid or art, strangely, for the heater" , "Domestic heater" , "It distributes heat in home, filled with hot water" .)