Except for being unique (4)

I believe the answer is:

only

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'unique' is the definition.
('only' can be a synonym of 'unique')

I cannot understand how the rest of the clue works.

(Other definitions for only that I've seen before include "And nothing more; sole" , "Mere" , "And no one else" , "Exclusive - individual" , "Except that; single" .)

I've seen this clue in the Evening Standard.

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