Flatfish; only (4)
I believe the answer is:
sole
I believe this is a double definition.
'flatfish' is the first definition.
(sole is a kind of flatfish)
'only' is the second definition.
(I know that only can be written as sole)
(Other definitions for sole that I've seen before include "Kind of fish - lose it" , "Exclusive class of fish" , "piece of leather" , "Underside of the foot" , "A lonely sort of flatfish" .)