Sound made by mobile form of nitrogen (8)
I believe the answer is:
ringtone
'sound made by mobile' is the definition.
'ringtone' can be an answer for 'sound' (I have seen 'Mobile sound ' mean 'ringtone' so perhaps 'sound' could also mean 'ringtone'). I'm unsure of the 'made by mobile' bit.
'form of nitrogen' is the wordplay.
'form of' indicates anagramming the letters (the letters need to be in another 'form').
'nitrogen' anagrammed gives 'RINGTONE'.
Can you help me to learn more?
(Other definitions for ringtone that I've seen before include "Sound heard when someone telephones" , "Sound made by a mobile phone" , "Nitrogen (anag.)" , "Sound of modern life" , "Sound coming from mobile" .)