Spirit of the air, or slim person (5)
I believe the answer is:
sylph
This clue is a double definition.
'spirit of the air' is the first definition.
(I've seen this in another clue)
'slim person' is the second definition.
(I've seen this before)
(Other definitions for sylph that I've seen before include "Imaginary spirit of the air" , "Slim woman" , "Thin and graceful female" , "Girl with slender figure" , "Slim person" .)