Most stable (9)
I believe the answer is:
steadiest
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">most stable</span>' is the definition.<br/ ><span class="explanation-format__explanation">('steady' can be similar in meaning to 'stable')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for steadiest that I've seen before include "Most reliable" , "Best-balanced -- it's sedate (anag)" , "Most secure" .)