Art collector in midst of assessing properties (7)
I believe the answer is:
estates
'properties' is the definition.
(estate is a kind of property)
'art collector in midst of assessing' is the wordplay.
'art collector' becomes 'tate' (UK institution organising four art museums).
'in' means one lot of letters goes inside another.
'midst of' indicates the central letters.
The middle letters of 'assessing' are 'ess'.
'tate' going inside 'ess' is 'ESTATES'.
(Other definitions for estates that I've seen before include "Property?" , "Large cars" , "lands for development" , "Demesnes" , "residential areas" .)