Quiet soldier turns to swine (3)
I believe the answer is:
pig
'swine' is the definition.
(pig is a kind of swine)
'quiet soldier turns' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'soldier' becomes 'gi' (term for US soldier).
'turns' is a reversal indicator.
'gi' backwards is 'ig'.
'p'+'ig'='PIG'
'to' acts as a link.
(Other definitions for pig that I've seen before include "Difficult task" , "Boar or sow" , "Domestic swine" , "Hog" , "person who eats too much" .)