Austere set are in City regularly (6)
I believe the answer is:
strict
'austere' is the definition.
(thesaurus)
'set are in city regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'set'+'are'+'in'+'city'='setareincity'
The alternating letters of 'setareincity' are 'STRICT'.
(Other definitions for strict that I've seen before include "Unbending, precise" , "Hard" , "Harsh and severe" , "Rigidly accurate" , "Demanding obedience to rules" .)