Not one to lose argument, raised voice (8)
I believe the answer is:
falsetto
'raised voice' is the definition.
(I've seen this before)
'not one to lose argument' is the wordplay.
'not' indicates named letters should be taken away.
'one' becomes 'i' (Roman numeral).
'to lose' becomes 'fail'.
'argument' becomes 'setto' (I've seen this before).
'fail' with 'i' taken away is 'fal'.
'fal'+'setto'='FALSETTO'
(Other definitions for falsetto that I've seen before include "High singing method" , "Way of singing at higher pitch" , "Unnaturally high-pitched voice" , "Male voice above the normal range" , "Voice pitch higher than the normal range" .)