Old Peruvians of a certain caste! (5)
I believe the answer is:
incas
'old peruvians' is the definition.
(I've seen this before)
'of a certain caste' is the wordplay.
'of' says the answer is hidden in the clue.
'INCAS' can be found hidden inside 'certain caste'.
(Other definitions for incas that I've seen before include "South Americans whose empire fell to Spain" , "Ancient inhabitants of Peru" , "conquered by Spain" , "Members of ancient Peruvian race" , "Cain's people in old Peru" .)