Air of mystery (4)
I believe the answer is:
aura
'air of mystery' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aura that I've seen before include "Nimbus" , "Distinctive but intangible quality surrounding a person" , "Air, distinctive quality" , "Particular ambience" , "'Air, atmosphere (4)'" .)