Soprano with energy, a singer and swimmer (3,4)
I believe the answer is:
Here is my best explanation:
'swimmer' is the definition.
(sea bass is a kind of fish)
'soprano with energy a singer' is the subsidiary indication.
'soprano' becomes 's' (musical abbreviation).
'with' says to put letters next to each other.
'energy' becomes 'e' (symbol in physics e.g. in Einstein's famous e=mca).
'singer' becomes 'bass' (bass is a kind of singer**).
'and' is the link.
(Other definitions for sea bass that I've seen before include "Fish", "Edible marine fish", "Saltwater fish", "Marine food fish", "one showing off plenty of scales" and "Food fish".)