From returning US soldiers, mother getting foreign letter (5)
I believe the answer is:
sigma
'foreign letter' is the definition.
(Greek letter)
'from returning us soldiers mother' is the wordplay.
I cannot quite understand how this works, but
'mother' could be 'ma' (Ma is another word for mother or mum) and 'ma' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'getting' is the link.
(Other definitions for sigma that I've seen before include "letter to Greeks" , "Greek letter used as a symbol for summation" , "The 18th letter of the Greek alphabet" , "Agism of the Greek letter" , "Greek character" .)