Crossword Genius

One turned to horse by a dramatist, she to something smaller (7)

Ross

I believe the answer is:

arachne

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'one' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see a link between them that I can't see?

I can't explain the remainder of the clue.

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(Other definitions for arachne that I've seen before include "She was transformed to a spider in Greek myth" , "Greek girl who unwisely challenged Athena and ended up as a spider" .)

I've seen this clue in The Times.
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