Petri-dish substance (4)
I believe the answer is:
agar
'petri-dish substance' 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 "used in making soups?" , "Jelly medium" , "Food-thickening jelly" , "contribution to culture?" , "that aids culture" .)