Contents of tin can once found in South America (4)
I believe the answer is:
inca
'once found in south america' is the definition.
(historical South American civilisation)
'contents of tin can' is the wordplay.
'contents of' says the answer is hidden in the clue.
'INCA' is hidden in the letters of 'tin can'.
(Other definitions for inca that I've seen before include "Old Peruvian descended from Cain" , "One of an old race of South America" , "Member of old Peruvian Indian race" , "Member of ancient Peruvian tribe" , "Ancient Peruvian Indian" .)