Frivolous — unstable (5)
I believe the answer is:
giddy
This clue is a double definition.
'frivolous' is the first definition.
(similar in meaning)
'unstable' is the second definition.
(thesaurus)
(Other definitions for giddy that I've seen before include "Suffering from vertigo" , "Sturdy" , "Dizzy or frivoloous" , "Frivolous -- liable to fall" , "Faint - excited" .)