Vegan's gelatin substitute (4)
I believe the answer is:
agar
'vegan's gelatin substitute' 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 "Substance derived from seaweed" , "found in the sea" , "Jelly medium" , "Food thickening agent obtained from seaweed" , "What comes out of seaweed" .)