Chemical processing hides missing gallons (6)
I believe the answer is:
tannin
'chemical' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Maybe you can see a link between them that I don't see?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for tannin that I've seen before include "Substance used in leather production" , "Vegetable substance used in making wine" , "Substance used in treating leather" , "It's found in tea and wine" , "There's something in red wine" .)