The most common and most likely answer to this crossword clue is the 6 letter word SIMIAN
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The straight part
We know that "who is his ancestor" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case who is his ancestor = simian.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 SIMIAN mean?
SIMIAN noun- An ape or monkey, especially an anthropoid.
- Of or pertaining to apes; apelike.
- Bearing resemblance to an ape.