Fish left lonesome at sea (5,4)
I believe the answer is:
lemon sole
'fish' is the definition.
(lemon sole is a kind of fish)
'left lonesome at sea' is the wordplay.
'left' becomes 'l' (common abbreviation).
'at sea' indicates anagramming the letters.
'lonesome' with letters rearranged gives 'emonsole'.
'l'+'emonsole'='LEMON SOLE'
(Other definitions for lemon sole that I've seen before include "swimmer" , "Tasty plaice of citrus origin?" , "Flatfish, valued as food" , "Edible flatfish" , "Common flatfish" .)