Sphere of physical mass (5)
I believe the answer is:
realm
'sphere' is the definition.
(synonyms)
'physical mass' is the wordplay.
'physical' becomes 'real' (I've seen this in other clues).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'real'+'m'='REALM'
'of' acts as a link.
(Other definitions for realm that I've seen before include "Kingdom, domain" , "Monarchy" , "Land under a king or queen" , "King's domain" , "A Nation" .)