A very quiet London borough is charming (9)
I believe the answer is:
appealing
'is charming' is the definition.
(I've seen this before)
'a very quiet london borough' is the wordplay.
'very quiet' becomes 'pp' (musical abbreviation for pianissimo).
'london borough' becomes 'ealing' (Ealing is a borough of London).
'a'+'pp'+'ealing'='APPEALING'
(Other definitions for appealing that I've seen before include "Attractive, interesting" , "Likeable or imploring" , "Making a heartfelt request" , "pleasing" , "Charming" .)