Quiet loan in resentful magnificence (9)
I believe the answer is:
splendour
'magnificence' is the definition.
(I know that magnificence can be written as splendour)
'quiet loan in resentful' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'loan' becomes 'lend' (synonyms).
'in' is an insertion indicator.
'resentful' becomes 'sour' (I can't justify this - if you can you should give a lot more credence to this answer).
'p'+'lend'='plend'
'plend' inserted inside 'sour' is 'SPLENDOUR'.
Can you help me to learn more?
(Other definitions for splendour that I've seen before include "Opulence" , "Grandeur" , "Spectacle" , "Sumptuous display - Len's proud (anag)" , "show" .)