Unique flatfish (4)

I believe the answer is:

sole

This clue is a double definition.

'unique' is the first definition.
(synonyms)

'flatfish' is the second definition.
(sole is a kind of flatfish)

(Other definitions for sole that I've seen before include "Kind of fish - lose it" , "O __ Mio (Neapolitan song)" , "European flatfish" , "One and only; part of foot" , "piece of leather" .)

I've seen this clue in The Sun.

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