The most common and most likely answer to this crossword clue is the 6 letter word CITRIC
C
I
T
R
I
C
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The straight part
We know that "acid" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case acid = citric.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CITRIC mean?
CITRIC adjective- Of, pertaining to, or derived from, the citron or lemon.