A key, say, as property (6)
I believe the answer is:
estate
'property' is the definition.
(I know that estate is a type of property)
'a key say' is the wordplay.
'a key' becomes 'e' (I've seen this in other clues).
'say' becomes 'state' (synonyms).
'e'+'state'='ESTATE'
'as' acts as a link.
(Other definitions for estate that I've seen before include "Rank - housing development - car" , "park perhaps" , "Piece of land with a lot of houses" , "All of one's assets after death" , "All that one owns at death" .)