Petri dish filler (4)
I believe the answer is:
agar
'petri dish filler' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for agar that I've seen before include "setter" , "Substance derived from seaweed" , "Seaweed jelly" , "Jelly medium" , "Seaweed derivative" .)