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 wordplay.
'river' becomes 'r'.
'in' indicates putting letters inside.
'shade of green?' becomes 'sage'.
'r' going inside 'sage' is 'SARGE'.
(Other definitions for sarge that I've seen before include "Local police officer", "NCO (colloq.)", "Police officer" and "Officer".)