Old language Zulu introduced to a private eye, perhaps (5)
I believe the answer is:
aztec
'old language' is the definition.
(language of the Aztec people)
'zulu introduced to a private eye perhaps' is the wordplay.
'zulu' becomes 'Z' (phonetic alphabet: alpha, bravo, charlie etc.).
'introduced to' indicates putting letters inside.
'private eye perhaps' becomes 'tec' (short for detective).
'a'+'tec'='atec'
'z' inserted within 'atec' is 'AZTEC'.
(Other definitions for aztec that I've seen before include "one of Mexican Indian extraction" , "Relating to old civilisation in Mexico" , "Member of ancient Mexican tribe" , "Former Mexican - Indian race" , "Conquered Mexican" .)