I can upset a Peruvian (4)
I believe the answer is:
inca
'a peruvian' is the definition.
(I've seen this before)
'i can upset' is the wordplay.
'upset' indicates anagramming the letters.
'can' with letters rearranged gives 'nca'.
'i'+'nca'='INCA'
(Other definitions for inca that I've seen before include "Inhabitant of old Peru" , "Pyramid builders" , "Member of old Peruvian Indian race" , "Ancient South American" , "Member of old South American Indian people of Peru" .)