An unknown detective finding Montezuma? (5)
I believe the answer is:
aztec
'montezuma?' is the definition.
(I've seen this in another clue)
'an unknown detective' is the wordplay.
'an' becomes 'a'.
'unknown' becomes 'z' (represents an unknown value in algebra).
'detective' becomes 'tec' (tec is short for detective).
'a'+'z'+'tec'='AZTEC'
'finding' acts as a link.
(Other definitions for aztec that I've seen before include "One into human sacrifice" , "One of Montezuma's people" , "Old empire of Mexico" , "Such as Moctezuma" , "Old Mexican empire" .)