Soldier turns in end of letter - swine! (4)
I believe the answer is:
pigs
'swine' is the definition.
(swine means pigs)
'soldier turns in end of letter' is the wordplay.
'soldier' becomes 'GI' (GI can mean a US soldier).
'turns' says the letters should be written in reverse.
'in' is an insertion indicator.
'end of letter' becomes 'ps' (I am not sure about this - if you are sure you should believe this answer much more).
'gi' written backwards gives 'ig'.
'ig' put into 'ps' is 'PIGS'.
Can you help me to learn more?
(Other definitions for pigs that I've seen before include "Boars and sows" , "Farm animals" , "Swine" , "earthenware crocks" .)