American magazine adopting British tone (6)
I believe the answer is:
timbre
'tone' is the definition.
(I've seen this before)
'american magazine adopting british' is the wordplay.
'american magazine' becomes 'time'.
'adopting' is an insertion indicator.
'british' becomes 'br'.
'time' enclosing 'br' is 'TIMBRE'.
(Other definitions for timbre that I've seen before include "colour we hear" , "Distinctive sound quality" , "Quality of voice, sounds wooden" , "Tone of voice - sounds wooden" , "Tone-colour" .)