Swell to fortissimo (4)
I believe the answer is:
toff
'swell' is the definition.
(wealthy or high-class person)
'to fortissimo' is the wordplay.
'fortissimo' becomes 'ff'.
'to'+'ff'='TOFF'
(Other definitions for toff that I've seen before include "One could be stuck up" , "Posh bloke (sl)" , "In slang, a posh person" , "Posh chap" , "Posh type" .)