Measure rather more covers (5)
I believe the answer is:
therm
'measure' is the definition.
(therm is a kind of measure)
'rather more covers' is the wordplay.
'covers' indicates the answer is hidden within the clue (I've seen 'covering' mean this).
'THERM' is hidden in the letters of 'rather more'.
(Other definitions for therm that I've seen before include "Unit of heat energy" , "Former gas supply unit" , "Unit of heat and gas measure" , "Gasman's unit" , "Indicator of heat" .)