Summoning signal (4)
I believe the answer is:
gong
'summoning signal' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gong that I've seen before include "Round metal plate struck to resound" , "Dinner instrument?" , "Medal; forty down" , "Help, benefit" , "Wordsworth's Boy; Dostoevsky's hero" .)