Hums rendition of Wasted Time (6,9)
I believe the answer is:
fourth dimension
'time' is the definition.
(synonyms)
'hums rendition of wasted' is the wordplay.
'wasted' indicates anagramming the letters.
'hums'+'rendition'+'of'='humsrenditionof'
'humsrenditionof' anagrammed gives 'FOURTH DIMENSION'.
(Other definitions for fourth dimension that I've seen before include "It may be described as time" , "feature of spacetime" .)