Kingdom concerned with metal on motorway (5)
I believe the answer is:
realm
'kingdom' is the definition.
(kingdom is a type of country or realm)
'concerned with metal on motorway' is the wordplay.
'concerned with' becomes 're' ('re' means regarding or about).
'metal' becomes 'Al' (chemical symbol for aluminium).
'on' means one lot of letters go next to another.
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
're'+'al'+'m'='REALM'
(Other definitions for realm that I've seen before include "Country" , "Empire - state" , "Kingdom, or field of activity" , "Lamer (anag.)" , "World" .)