Become semi-solid on cooling (7)
I believe the answer is:
congeal
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">become semi-solid on cooling</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 congeal that I've seen before include "Set - go clean (anag)" , "Coagulate" , "Jell" , "cake" , "Become solid" .)