Flatfish being the only one (4)
I believe the answer is:
sole
This clue is a double definition.
'flatfish' is the first definition.
(sole is a kind of flatfish)
'being the only one' is the second definition.
(Other definitions for sole that I've seen before include "Unique" , "Fish - alone" , "fish > that's never less than foot in length" , "O __ Mio (Neapolitan song)" , "A lone sort of fish - lose it" .)