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As the saying goes, there's nothing I'm sharp about shifting (8)

Ross

I believe the answer is:

aphorism

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

I don't understand how the rest of the clue works.

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(Other definitions for aphorism that I've seen before include "One could briefly say this" , "Terse saying" , "A short, pithy, instructive saying" , "Short, clever saying" , "Brief pithy saying" .)

I've seen this clue in the Evening Standard.
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