One's heard on the phone offering to negotiate, to some extent (8)
I believe the answer is:
ringtone
'one's heard on the phone' is the definition.
'offering to negotiate to some extent' is the wordplay.
'to some extent' indicates the answer is hidden within the clue.
'RINGTONE' is hidden within 'offering to negotiate'.
(Other definitions for ringtone that I've seen before include "Sound of a phone on an incoming call" , "announcement of caller" , "Sound made by a mobile phone" , "Nitrogen (anag.)" , "Sound of modern life" .)