Is in possession of a will which mentions the primary property (7)
I believe the answer is:
testate
'a will which mentions the primary property' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'is in possession' is the wordplay.
'is in' becomes 't' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'possession' becomes 'estate' (estate is a kind of possession).
't'+'estate'='TESTATE'
'of' is the link.
Can you help me to learn more?
(Other definitions for testate that I've seen before include "Person willing" , "Having made a legal will" , "With a legally valid will" , "Having made a valid will" , "Having made a last will and testament" .)