Gelatinous substance from a fish? A fish!

I believe the answer is:

agar-agar

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(Other definitions for agar-agar that I've seen before include "Food thickener from seaweed" , "Jelly from seaweed" , "Gelatinous thickening for food" , "Thickener obtained from seaweed" .)

I've seen this clue in The Telegraph.

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