Concrete mass in field (5)
I believe the answer is:
realm
'field' is the definition.
(I've seen this before)
'concrete mass' is the wordplay.
'concrete' becomes 'real' ('real' can be similar in meaning to 'concrete').
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'real'+'m'='REALM'
'in' is the link.
(Other definitions for realm that I've seen before include "Kingdom could be lamer" , "Kingdom, domain" , "Kingdom or sphere" , "King's domain" , "Province" .)