Tungsten cube's mass (6)
I believe the answer is:
weight
'mass' is the definition.
(thesaurus)
'tungsten cube's' is the wordplay.
'tungsten' becomes 'w' ('W' can be a synonym of 'tungsten').
'cube' becomes 'eight' (8 is a cube number).
'w'+'eight'='WEIGHT'
(Other definitions for weight that I've seen before include "Attach importance to" , "Avoir-du-pois" , "Feeling of oppression; heaviness" , "Something to lose" , "Importance, oppressive force" .)