Quality of Britain admitted by the enemy (6)
I believe the answer is:
timbre
'quality' is the definition.
('timbre' can be a synonym of 'quality')
'britain admitted by the enemy' is the wordplay.
'britain' becomes 'br' (abbreviation).
'admitted by' means one lot of letters goes inside another.
'the enemy' becomes 'time' (the enemy when speaking figuratively).
'br' inserted inside 'time' is 'TIMBRE'.
'of' is the link.
(Other definitions for timbre that I've seen before include "Tone of voice - sounds wooden" , "Musical tone quality" , "Characteristic voice sound" , "Distinctive tone" , "The quality of the speaking voice" .)