Gel made from seaweed (4)
I believe the answer is:
agar
'gel made from seaweed' 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 "Seaweed derivative" , "part of culture?" , "Seaweed jelly" , "Jelly medium" , "used in making soups?" .)