Unique fish (4)
I believe the answer is:
sole
I believe this clue is a double definition.
'unique' is the first definition.
(synonyms)
'fish' is the second definition.
(I know that sole is a type of fish)
(Other definitions for sole that I've seen before include "Lone; part of foot" , "Dover - -" , "Bottom of the foot" , "swimmer" , "Fish - alone" .)