Crossword Genius

Could a toad sit on one? (5)

Ross

I believe the answer is:

stool

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'one?' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

I cannot understand how the remainder of the clue works.

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(Other definitions for stool that I've seen before include "Motion" , "Backless chair" , "Seat without a back" , "place butt on one?" , "Three-legged backless seat" .)

I've seen this clue in the Evening Standard.
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