Move together carrying light metal trumpet (6)
I believe the answer is:
'trumpet' is the definition.
'move together carrying light metal' is the wordplay.
'move together' becomes 'herd' (herding is a kind of moving together).
'carrying' is an insertion indicator.
'light metal' becomes 'al' (chemical symbol for Aluminium).
'herd' placed around 'al' is 'HERALD'.
(Other definitions for herald that I've seen before include "Bringer of news" , "One making proclamations - harbinger" , "Scottish newspaper" , "Harbinger, messenger" , "Proclaim the approach of, like announcer of old" .)