Gelatin is in vitreous vessel (9)
I believe the answer is:
isinglass
'gelatin' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Maybe there's a link between them I don't understand?
'is in vitreous vessel' is the wordplay.
'vitreous vessel' becomes 'glass' (I can't justify this - if you can you should give a lot more credence to this answer).
'is'+'in'+'glass'='ISINGLASS'
Can you help me to learn more?
(Other definitions for isinglass that I've seen before include "What's got from fish" , "Some Muscovite" , "Mineral" , "Fish gelatin" , "Gelatine" .)