One isn't convinced Conservative with sex appeal and muscles could be returned (7)

I believe the answer is:

sceptic

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

I can't explain the remainder of the clue.

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(Other definitions for sceptic that I've seen before include "One who doubts accepted opinions" , "Greek philosopher" , "A doubting Thomas" , "Someone inclined to disbelieve" , "Doubter; atheist" .)

I've seen this clue in The Independent.

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