The only fish (4)
I believe the answer is:
sole
I believe this clue is a double definition.
'the only' is the first definition.
(I've seen this before)
'fish' is the second definition.
(I know that sole is a type of fish)
(Other definitions for sole that I've seen before include "Unique" , "Kind of fish - lose it" , "Just one - fish" , "fish in the sea!" , "fish > that's never less than foot in length" .)