Quality of magazine about Britain (6)
I believe the answer is:
Here is my best explanation:
'quality' is the definition.
('timbre' can be a synonym of 'quality')
'magazine about britain' is the subsidiary indication.
'magazine' becomes 'time' (US magazine).
'about' is an insertion indicator.
'britain' becomes 'br' (abbreviation).
'time' placed around 'br' is 'timbre'.
'of' is the link.
(Other definitions for timbre that I've seen before include "Voice quality", "(A sound's) character", "Quality of tone", "Sound's character", "Musical tone quality", "Distinctive sound quality", "A quality of sound", "colour we hear", "Character (of sound)" and "Tone of voice - sounds wooden".)