Belief Area 51's infiltrated (8)
I believe the answer is:
religion
'belief' is the definition.
(I know that religion is a type of belief)
'area 51's infiltrated' is the wordplay.
'area' becomes 'region' (synonyms).
'51' becomes 'LI' (Roman numerals).
'infiltrated' means one lot of letters goes inside another.
'region' placed around 'li' is 'RELIGION'.
(Other definitions for religion that I've seen before include "System of divine worship" , "Institution to express belief in a divine power" , "Particular system of belief" , "Marx's pium of the people" , "Belief - way of life of monks and nuns" .)