Sailor's back on ship pursuing ocean fish (3,4)
I believe the answer is:
sea bass
'fish' is the definition.
(I know that sea bass is a type of food fish)
'sailor's back on ship pursuing ocean' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'back' shows that the letters should be reversed in order.
'on ship' becomes 'ss' (prefix in ship names eg SS Great Britain. I am not sure about the 'on' bit.).
'pursuing' is a charade indicator (letters next to each other).
'ocean' becomes 'sea' (synonyms).
'ab' back-to-front is 'ba'.
'ba'+'ss'='bass'
'bass' put after 'sea' is 'SEA-BASS'.
(Other definitions for sea bass that I've seen before include "Marine food fish" , "one showing off plenty of scales" , "Saltwater fish" , "Edible marine fish" .)