Sphere's actual mass (5)
I believe the answer is:
realm
'sphere's' is the definition.
(synonyms)
'actual mass' is the wordplay.
'actual' becomes 'real' (synonyms).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'real'+'m'='REALM'
(Other definitions for realm that I've seen before include "Royal domain" , "field of study" , "Province" , "Kingdom, domain" , "Kingdom could be lamer" .)