Old heating unit bust after time (5)
I believe the answer is:
therm
'old heating unit' is the definition.
(old-fashioned unit of heat)
'bust after time' is the wordplay.
'bust' becomes 'herm' (herm is a type of statue or bust).
'after' means one lot of letters go next to another.
'time' becomes 't' (abbreviation).
'herm' put after 't' is 'THERM'.
(Other definitions for therm that I've seen before include "Amount of heat" , "it measures heat" , "Unit of heat energy" , "Unit of heat and gas measure" , "A hot unit" .)