Quality of a sound medic, cutting fag out (6)
I believe the answer is:
timbre
'quality of a sound' is the definition.
(I've seen this before)
'medic cutting fag out' is the wordplay.
'medic' becomes 'MB' (Bachelor of Medicine).
'cutting' is an insertion indicator (some letters cut their way into a word).
'fag out' becomes 'tire' (synonyms).
'mb' placed inside 'tire' is 'TIMBRE'.
(Other definitions for timbre that I've seen before include "Characteristic quality of sound" , "Distinctive sound quality" , "Ring" , "Sound's character" , "Characteristic voice sound" .)