Copper river in shade of green? (5)
I believe the answer is:
Here is my best explanation:
'copper' is the definition.
(informal way of addressing a police sergeant)
'river in shade of green?' is the subsidiary indication.
'river' becomes 'r'.
'in' is an insertion indicator.
'shade of green?' becomes 'sage'.
'r' going into 'sage' is 'sarge'.
(Another definition for sarge that I've seen is "NCO (colloq.)".)