Certain something (2,2,4,4)
(Other definitions for je ne sais quoi that I've seen before include "Elusive but pleasing quality" , "(A certain) something" , "Quality that can't be easily described" , "Something indescribable (French)" , "Indefinable quality" .)
'certain something' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for aura that I've seen before include "Intangible atmosphere" , "Mystic air" , "Indiscernible quality" , "Atmosphere [ital]" , "Air, distinctive quality" .)