Solid, stable (4)
I believe the answer is:
firm
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">solid</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that solid can be written as firm)</span><br/ ><br/ >'<span class="explanation-format__original">stable</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ ></span>
(Other definitions for firm that I've seen before include "Company - solid" , "'Solid, unyielding (4)'" , "unlikely to collapse" , "Resolved and steady" , "Steadfast" .)