Article placed on top of room unit (5)
I believe the answer is:
therm
'unit' is the definition.
(therm is a kind of unit)
'article placed on top of room' is the wordplay.
'article' becomes 'the' (the definite article).
'placed on top of' is a charade indicator (letters next to each other) (in a down clue, some letters go on top of others).
'room' becomes 'rm' (abbreviation for room).
'the'+'rm'='THERM'
(Other definitions for therm that I've seen before include "Unit of heat used in the UK as the basis of gas bills" , "Unit of heat energy" , "Unit of heat as in gas supply" , "Former gas supply unit" , "Unit of heat and gas measure" .)