The most common and most likely answer to this crossword clue is the 10 letter word ABORIGINAL
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The straight part
We know that "Earliest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case earliest = aboriginal.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 ABORIGINAL mean?
ABORIGINAL noun- An Aboriginal inhabitant of Australia, Aborigine.
- Of or pertaining to Australian Aboriginal peoples, Aborigines, or their language.
- An animal or plant native to a region.
- First according to historical or scientific records; original; indigenous; primitive.
- Living in a land before colonization by the Europeans.