Quality of magazine about Britain (6)
I believe the answer is:
timbre
'quality' is the definition.
('timbre' can be a synonym of 'quality')
'magazine about britain' is the wordplay.
'magazine' becomes 'time' (US magazine).
'about' is an insertion indicator.
'britain' becomes 'br' (abbreviation).
'time' going around 'br' is 'TIMBRE'.
'of' is the link.
(Other definitions for timbre that I've seen before include "Musical tone quality" , "Character (of sound)" , "Quality of sound of the voice" , "Characteristic quality of sound" , "Characteristic voice sound" .)