Soprano with energy, a singer and swimmer (3,4)
I believe the answer is:
sea bass
'swimmer' is the definition.
(sea bass is a kind of fish)
'soprano with energy a singer' is the wordplay.
'soprano' becomes 's' (musical abbreviation).
'with' says to put letters next to each other.
'energy' becomes 'e' (physics symbol as in E=mca).
'singer' becomes 'bass' (bass is a kind of singer**).
's'+'e'+'a'+'bass'='SEA-BASS'
'and' is the link.
(Other definitions for sea bass that I've seen before include "Marine food fish" , "Edible marine fish" , "one showing off plenty of scales" , "Saltwater fish" .)