Swine, the boxer, having change of heart (3)
I believe the answer is:
pig
'swine' is the definition.
(pig is a kind of swine)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for pig that I've seen before include "Hog" , "Farmyard animal" , "Porcine animal" , "Eat greedily" , "person who eats too much" .)