The most common and most likely answer to this crossword clue is the 12 letter word ADVENTITIOUS
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The straight part
We know that "accidental" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case accidental = adventitious.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
ADVENTITIOUS adjective- From an external source; not innate or inherent, foreign.
- Accidental, additional, appearing casually.
- Not congenital; acquired.
- Developing in an unusual place or from an unusual source.