Poor child's letter from abroad found in container (6)
I believe the answer is:
urchin
'poor child's' is the definition.
(urchins are poor children)
'letter from abroad found in container' is the wordplay.
'letter from abroad' becomes 'chi' (letter of the Greek alphabet).
'found in' indicates putting letters inside.
'container' becomes 'urn' (urn is a kind of container).
'chi' put inside 'urn' is 'URCHIN'.
(Other definitions for urchin that I've seen before include "Ragged child; marine animal" , "grubby kid" , "Ragged child or sea creature with round shell" , "Young troublemaker" , "seafood for some" .)