Mobile resonating as missing such a noise (8)
I believe the answer is:
ringtone
'a noise' is the definition.
(noise from a phone)
'mobile resonating as missing' is the wordplay.
'mobile' indicates an anagram.
'missing' indicates named letters should be taken away.
'resonating' with 'as' removed is 'reonting'.
'reonting' with letters rearranged gives 'RINGTONE'.
'such' is the link.
(Other definitions for ringtone that I've seen before include "Sound heard when someone telephones" , "announcement of caller" , "Nitrogen (anag.)" , "Sound of a phone on an incoming call" , "Sound coming from mobile" .)