Article on empty space unit (5)
I believe the answer is:
therm
'unit' is the definition.
(therm is a kind of unit)
'article on empty space' is the wordplay.
'article' becomes 'the' (the definite article in English).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'empty' means to remove the middle letters.
'space' becomes 'room' (enough space or room to do something).
'room' with its centre taken out is 'rm'.
'the'+'rm'='THERM'
(Other definitions for therm that I've seen before include "Unit of heat as in gas supply" , "Former gas supply unit" , "Indicator of heat" , "Unit of heat and gas measure" , "Unit of measurement of heat" .)