Cain's upset some Peruvians (5)
I believe the answer is:
incas
'peruvians' is the definition.
(I've seen this in another clue)
'cain's upset' is the wordplay.
'upset' indicates anagramming the letters.
'cains' is an anagram of 'INCAS'.
'some' is the link.
(Other definitions for incas that I've seen before include "Members of ancient Peruvian race" , "Old South American royals" , "Old Peruvians" , "Ancient Peruvians" , "Old people coming from South America" .)