The most common and most likely answer to this crossword clue is the 6 letter word ARCHIL
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The straight part
We know that "Violet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case violet = archil.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.
Crossword Answer definitions
ARCHIL noun- Orchil, a violet dye obtained from several species of lichen (Roccella tinctoria, etc.), which grow on maritime rocks in the Canary and Cape Verde Islands, etc.
- The plant from which the dye is obtained.