Fish alone (4)
I believe the answer is:
sole
This clue is a double definition.
'fish' is the first definition.
(I know that sole is a type of fish)
'alone' is the second definition.
(synonyms)
(Other definitions for sole that I've seen before include "A fish; part of shoe" , "O __ Mio (Neapolitan song)" , "fish in the sea!" , "One and only; part of foot" , "piece of leather" .)