Sanctuary unknown in a degraded area (6)
I believe the answer is:
asylum
'sanctuary' is the definition.
('asylum' can be a synonym of 'sanctuary')
'unknown in a degraded area' is the wordplay.
'unknown' becomes 'y' (algebra).
'in' indicates putting letters inside.
'degraded area' becomes 'slum'.
'a'+'slum'='aslum'
'y' put into 'aslum' is 'ASYLUM'.
(Other definitions for asylum that I've seen before include "Institution for care of the insane" , "Sanctuary or institution for mentally ill" , "Refugee's protection" , "Refuge or home for mentally ill" , "Place of refuge, once a mental institution" .)