Lightweight (4-5)
I believe the answer is:
airy-fairy
'lightweight' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for airy-fairy that I've seen before include "Unrealistic - fanciful" , "Fanciful or impractical for breezy elf" , "Lightweight" , "Fanciful and impractical like breezy sprite" , "Fanciful, not based on reality" .)