Swimmer Guy, very soft at heart (5)
I believe the answer is:
guppy
'swimmer' is the definition.
(guppy is a kind of fish)
'guy very soft at heart' is the wordplay.
'very soft' becomes 'pp' (musical abbreviation for pianissimo).
'at heart' indicates putting letters inside (inserted letters go to the heart of the word).
'guy' enclosing 'pp' is 'GUPPY'.
(Other definitions for guppy that I've seen before include "Tropical fish" , "Small fish of northern South America and the Caribbean" , "Freshwater fish" , "Tank occupant" , "Aquarium fish" .)