Language of Zulu arrested by an investigating officer (5)
I believe the answer is:
aztec
'language' is the definition.
(language of the Aztec Empire)
'zulu arrested by an investigating officer' is the wordplay.
'zulu' becomes 'Z' (phonetic alphabet: alpha, bravo, charlie etc.).
'arrested by' means one lot of letters goes inside another (I've seen this in other clues).
'an' becomes 'a'.
'investigating officer' becomes 'tec' (short for detective).
'a'+'tec'='atec'
'z' going into 'atec' is 'AZTEC'.
'of' is the link.
(Other definitions for aztec that I've seen before include "One of Montezuma's people" , "Like Montezuma?" , "Native of old Mexico" , "nickname of US State" , "Person of earlier Mexican civilization" .)