Property describing single truth (7)
I believe the answer is:
reality
'truth' is the definition.
(I've seen this in another clue)
'property describing single' is the wordplay.
'property' becomes 'real property' (real property is a kind of property).
'describing single' becomes 'ity' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'real'+'ity'='REALITY'
Can you help me to learn more?
(Other definitions for reality that I've seen before include "Actual existence" , "All that is actual and not imaginary" , "It's faced by pragmatists" , "Truth, actuality" , "as some TV programmes purport to show" .)