Crossword Genius

Not the only word of exception (3)

Ross

I believe the answer is:

but

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'not' is the definition.
Both the definition and answer are adverbs. Perhaps they are linked in a way I don't understand?

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

Can you help me to learn more?

(Other definitions for but that I've seen before include "Save" , "Nevertheless, on the other hand" , "Except; nevertheless" , "On the contrary" , "Only; apart from" .)

I've seen this clue in the Evening Standard.
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