Become more solid (6)
I believe the answer is:
harden
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">become more solid</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for harden that I've seen before include "Become less soft" , "Become stronger or firmer" , "Set; become tougher" , "Small fly" , "Temper, steel say" .)