Become solid (3)
(Other definitions for set that I've seen before include "Customary < scenery < a TV < series" , "Scenery for play or film" , "decline" , "Hardened" , "Major unit of tennis match" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">become solid</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for gel that I've seen before include "Hair styling substance" , "Viscid substance" , "Upstage" , "Hair-dressing preparation" , "Hair cosmetic" .)