As the saying goes, there's nothing I'm sharp about shifting (8)
I believe the answer is:
aphorism
'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.
Can you help me to learn more?
(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" .)