Field that has concrete mass (5)
I believe the answer is:
realm
'field' is the definition.
(synonyms)
'concrete mass' is the wordplay.
'concrete' becomes 'real' (similar in meaning).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'real'+'m'='REALM'
'that has' acts as a link.
(Other definitions for realm that I've seen before include "monarch's responsibility" , "Knowledge base" , "Kingdom could be lamer" , "Kingdom's" , "Peers are possessed by this" .)