The only one afoot? (4)

I believe the answer is:

sole

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'the only one' is the definition.
(I've seen this in another clue)

I cannot understand how the rest of the clue works.

(Other definitions for sole that I've seen before include "Shoe part -- lone" , "European flatfish" , "Underside of the foot" , "A lonely sort of flatfish" , "fish in the sea!" .)

I've seen this clue in the Evening Standard.

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