Happy notes mixed up in bag (9)
I believe the answer is:
gladstone
'bag' is the definition.
(Gladstone is an example)
'happy notes mixed up in' is the wordplay.
'happy' becomes 'glad' (similar in meaning).
'mixed up' indicates an anagram.
'in' is an insertion indicator.
'notes' anagrammed gives 'stone'.
'glad' inserted into 'stone' is 'GLADSTONE'.
(Other definitions for gladstone that I've seen before include "British statesman, four times PM, d. 1898" , "19th century PM" , "Old English P.M. or a doctor's bag" , "5,27 [PRIME MINISTER]" , "Victorian GOM" .)