Petri dish filler (4)
I believe the answer is:
agar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">petri dish filler</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 agar that I've seen before include "setter" , "Substance derived from seaweed" , "Seaweed jelly" , "Jelly medium" , "Seaweed derivative" .)