Petri dish gelatin (4)
I believe the answer is:
agar
'petri dish gelatin' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for agar that I've seen before include "Seaweed jelly" , "Food-thickening jelly" , "Jelly got from seaweeds, used in foods" , "found in the sea" , "What comes out of seaweed" .)