Some gelatin is bottled (9)

I believe the answer is:

isinglass

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But I can't explain it at all! Could you help me understand?

(Other definitions for isinglass that I've seen before include "Lass, I sing badly about this preservative" , "fish produce" , "Fish gelatin" , "Beer finings" , "Fish product used in brewing" .)

I've seen this clue in the Sydney Morning Herald.

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