Peruvian in Spain cannot be understood (4)
I believe the answer is:
inca
'peruvian' is the definition.
(the Inca were a historical people of Peru)
'in spain cannot be understood' is the wordplay.
'in' indicates a hidden word.
'INCA' is hidden within 'spain cannot understood'.
(Other definitions for inca that I've seen before include "Pre-conquest Andean" , "An old Peruvian like Cain" , "Atahualpa was" , "Member of old Peruvian Indian race" , "Ancient civilisation of Peru" .)