Only fish (4)
I believe the answer is:
sole
I believe this clue is a double definition.
'only' is the first definition.
(synonyms)
'fish' is the second definition.
(sole is a kind of fish)
(Other definitions for sole that I've seen before include "swimmer" , "piece of leather" , "Exclusive class of fish" , "Fish; part of foot" , "Flatfish; exclusive" .)