Aforementioned copper since retired (4)
I believe the answer is:
said
'aforementioned' is the definition.
(I know that aforementioned can be written as said)
'copper since retired' is the wordplay.
'copper' becomes 'DI' (detective inspector, type of copper).
'since' becomes 'as' (I've seen this in other clues).
'retired' says the letters should be written in reverse.
'di'+'as'='dias'
'dias' backwards is 'SAID'.
(Other definitions for said that I've seen before include "Announced; Egypt. Port" , "Remarked" , "Spoke" , "Averred" , "... gave expression" .)