Seaweed gelatin (4)
I believe the answer is:
agar
'seaweed 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 "setter" , "found in the sea" , "Food thickening agent obtained from seaweed" , "Seaweed derivative" , "Food-thickening jelly" .)