Sound quality of Britain reduced in time (6)
I believe the answer is:
timbre
'sound quality' is the definition.
(I've seen this before)
'britain reduced in time' is the wordplay.
'britain reduced' becomes 'br' (abbreviation; 'reduced' asks for an abbreviation).
'in' is an insertion indicator.
'br' going into 'time' is 'TIMBRE'.
'of' is the link.
(Other definitions for timbre that I've seen before include "Quality of sound of the voice" , "Musical tone quality" , "Distinctive sound quality" , "Characteristic voice sound" , "Quality of voice, sounds wooden" .)