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