Only flat fish (4)
I believe the answer is:
sole
This clue is a double definition.
'only' is the first definition.
(I know that only can be written as sole)
'flat fish' is the second definition.
(I've seen this in another clue)
(Other definitions for sole that I've seen before include "Exclusive class of fish" , "Just one - fish" , "piece of leather" , "Kind of fish - lose it" , "swimmer" .)