Single fish (4)
I believe the answer is:
sole
I believe this clue is a double definition.
'single' 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 "Unique" , "swimmer" , "Fish; part of foot" , "Lone; part of foot" , "Dover - -" .)