Left in empty trumpet (5)
I believe the answer is:
blare
'trumpet' is the definition.
(I've seen this in another clue)
'left in empty' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'empty' becomes 'bare' (eg 'the cupboards were bare').
'l' placed into 'bare' is 'BLARE'.
(Other definitions for blare that I've seen before include "Loud harsh noise" , "Sound loudly" , "Sound loud and harsh" , "Strident sound" , "Roar" .)