The most common and most likely answer to this crossword clue is the 9 letter word ANCESTORS
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Buddy explains!
The straight part
We know that "long-lost relatives" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case long-lost relatives = ancestors.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 ANCESTORS mean?
ANCESTORS noun- One from whom a person is descended, whether on the father's or mother's side, at any distance of time; a progenitor; a forefather.
- An earlier type; a progenitor
- One from whom an estate has descended;—the correlative of heir.
- One who had the same role or function in former times.
- A word or phrase which serves as the origin of a term in another language.
- To be an ancestor of.