Not so well (5)

I believe the answer is:

worse

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'not so well' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I can't see?

This is the entire clue.

Can you help me to learn more?

(Other definitions for worse that I've seen before include "Bad to a greater degree" , "More serious" , "Of poorer quality" , "More awful or unwell" , "Opposite of better" .)

I've seen this clue in The Guardian and The Telegraph.

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