Solid, firm (7)
I believe the answer is:
compact
<span class="explanation-format__ClueExplanation">I believe 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">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">firm</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ ></span>
(Other definitions for compact that I've seen before include "Small and neat - agreement" , "Make denser, agreement" , "Dense; treaty" , "'Condensed, portable (7)'" , "Dense; agreement" .)