Key to national property (6)
I believe the answer is:
estate
'property' is the definition.
(estate is a kind of property)
'key to national' is the wordplay.
'key' becomes 'e' (I've seen this in other clues).
'to national' becomes 'state' (I am not sure about this - if you are sure you should believe this answer much more).
'e'+'state'='ESTATE'
Can you help me to learn more?
(Other definitions for estate that I've seen before include "Property with large grounds" , "Place for tea or coffee" , "Property - tea set (anag)" , "Houses - car" , "Property - may go with rank" .)