Gel made from seaweed (4)
I believe the answer is:
agar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gel made from seaweed</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for agar that I've seen before include "Seaweed derivative" , "part of culture?" , "Seaweed jelly" , "Jelly medium" , "used in making soups?" .)