A study curtailed in district (4)
I believe the answer is:
area
'district' is the definition.
(thesaurus)
'a study curtailed' is the wordplay.
'study' becomes 'read' ('read' can be a synonym of 'study').
'curtailed' means to remove the last letter.
'read' with its last letter taken off is 'rea'.
'a'+'rea'='AREA'
'in' acts as a link.
(Other definitions for area that I've seen before include "topic" , "Field -- expanse" , "The measure of a surface" , "Extent of a 2-dimensional surface" , "Expanse of a surface" .)