A bit of weather must heat unit (5)
I believe the answer is:
therm
'heat unit' is the definition.
(therm is a kind of heat unit)
'a bit of weather must' is the wordplay.
'a bit of' indicates the answer is hidden within the clue (I've seen 'a bit' mean this (the answer is formed by a bit of the clue)).
'THERM' is hidden within 'weather must'.
(Other definitions for therm that I've seen before include "Unit of heat as in gas supply" , "Indicator of heat" , "Former gas supply unit" , "Unit of measurement of heat" , "Unit of heat and gas measure" .)