Only; flatfish (4)
I believe the answer is:
sole
I believe this is a double definition.
'only' is the first definition.
(I know that only can be written as sole)
'flatfish' is the second definition.
(sole is a kind of flatfish)
(Other definitions for sole that I've seen before include "swimmer" , "fish in the sea!" , "Lone; part of foot" , "Shoe part -- lone" , "Kind of fish - lose it" .)