Area 51's not part of belief system (6)
I believe the answer is:
region
'area' is the definition.
(I know that area can be written as region)
'51's not part of belief system' is the wordplay.
'51' becomes 'LI' (Roman numerals).
'not part of' is a deletion indicator.
'belief system' becomes 'religion' (I've seen this before).
'religion' with 'li' removed is 'REGION'.
(Other definitions for region that I've seen before include "Part of a country" , "Ignore (anag) - area" , "Area, zone" , "Area - district" , "Ignore (anag) -- expanse of land" .)