Gelatin made from seaweed (4)
I believe the answer is:
agar
'gelatin made from seaweed' 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 thickening agent obtained from seaweed" , "Food-thickening jelly" , "a newspaper rejected" , "Food thickener (derived from seaweed)" , "Jelly used in bacteria culture" .)