Swimmer very quietly punches man (5)
I believe the answer is:
guppy
'swimmer' is the definition.
(guppy is a kind of fish)
'quietly punches man' is the wordplay.
'quietly' becomes 'pp' (musical abbreviation for pianissimo).
'punches' indicates putting letters inside (punch can mean to create a hole in).
'man' becomes 'guy' (guy is a kind of man).
'pp' going inside 'guy' is 'GUPPY'.
'very' acts as a link.
(Other definitions for guppy that I've seen before include "Tank occupant" , "Aquarium fish" , "Freshwater fish" , "Small fish of northern South America and the Caribbean" , "Tropical fish" .)