The most common and most likely answer to this crossword clue is the 8 letter word TETRAPOD
I've seen this crossword clue in The Times
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More clues leading to the result TETRAPOD
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The straight part
We know that "Horse for one" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case horse for one = tetrapod.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 TETRAPOD mean?
TETRAPOD noun- Any vertebrate with four limbs.
- Any vertebrate (such as birds or snakes) that has evolved from early tetrapods; especially any member of the superclass Tetrapoda
- A concrete structure with arms, used to arrest wave energy along the shore in sea defence projects.
- Having four limbs or feet