Classic tune really, although different originally (4)
I believe the answer is:
trad
'classic' is the definition.
(abbreviation of traditional)
'tune really although different originally' is the wordplay.
'originally' indicates taking the first letters.
The first letters of 'tune really although different' is 'TRAD'.
(Other definitions for trad that I've seen before include "Form of folk or jazz" , "Style of jazz" , "music" , "Type of jazz or folk" , "(Of jazz) traditional" .)