Officers' instruments (5)
I believe the answer is:
brass
'instruments' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for brass that I've seen before include "Gall" , "Nerve (but not of steel!)" , "Zinc-copper alloy - cheek" , "Yellowish alloy" , "cash" .)