Biology lab gel (4)
I believe the answer is:
agar
'biology lab gel' 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 "Food thickener (derived from seaweed)" , "a newspaper rejected" , "Food thickening agent obtained from seaweed" , "Food-thickening jelly" , "Jelly medium" .)