Gelatin substitute (4)
I believe the answer is:
agar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gelatin substitute</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 "found in the sea" , "Food-thickening jelly" , "Medium for growth" , "setter" , "Jelly used in bacteria culture" .)