Brass instrument; plant (5)
I believe the answer is:
bugle
This clue is a double definition.
'brass instrument' is the first definition.
(bugle is a type of brass instrument)
'plant' is the second definition.
(also known as bugleweed)
(Other definitions for bugle that I've seen before include "Small trumpet; plant" , "Brass instrument without valves" , "A military trumpet" , "'Last Post' instrument" , "Brass instrument; plant" .)