Crossword Genius

Tartaric acid? (4)

Ross

I believe the answer is:

tart

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This might be a double definition.

'tartaric' is the first definition.
Although both the answer and definition are adjectives, I cannot understand how one could define the other.

'acid?' is the second definition.
(thesaurus)

Can you help me to learn more?

(Other definitions for tart that I've seen before include "Of taste, sharp or acid" , "The pastry sounds bitter" , "sour type of pie" , "A bit sour like open cake" , "Sharp - bakery product" .)

I've seen this clue in the Evening Standard.
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