Huge mobile amplifier (9)
I believe the answer is:
megaphone
'amplifier' is the definition.
(I've seen this before)
'huge mobile' is the wordplay.
'huge' becomes 'mega' (mega can mean very large).
'mobile' becomes 'phone' (mobile is a type of phone).
'mega'+'phone'='MEGAPHONE'
(Other definitions for megaphone that I've seen before include "Cone-shaped device to intensify the human voice" , "hearing aid" , "Funnel-shaped voice amplifier" , "form of diplomacy?" , "One projects sound" .)