Silly – light-headed (5)
I believe the answer is:
giddy
I believe this clue is a double definition.
'silly' is the first definition.
(I know that silly can be written as giddy)
'light-headed' is the second definition.
(I know that light-headed can be written as giddy)
(Other definitions for giddy that I've seen before include "Dizzy; frivolous" , "Sturdy" , "Frivolous -- liable to fall" , "Frivolous - in a spin?" , "Dizzy - flighty" .)