Shrill trumpet (7)
I believe the answer is:
clarion
'shrill trumpet' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for clarion that I've seen before include "Loud and clear; shrill war trumpet" , "Kind of trumpet" , "Trumpet (now more likely a newspaper)" , "Medieval brass instrument heard in carol" , "Medieval brass instrument with a clear shrill tone" .)