Old people as outspoken investigators (6)
I believe the answer is:
aztecs
'old people' is the definition.
'Aztecs' can be an answer for 'people' (Aztec is an example). I am not certain of the 'old' bit.
'outspoken investigators' is the wordplay.
'outspoken' becomes 'az' (I can't explain this - if you can you should believe this answer much more).
'investigators' becomes 'tecs' ('tec' is short for detective).
'az'+'tecs'='AZTECS'
'as' is the link.
(Other definitions for aztecs that I've seen before include "Ancient Mexicans" , "Mexican Indians of old" , "Early Mexicans" , "race" , "Indian people of Mexico before Spanish conquest" .)