Had note about unknown composer (5)
I believe the answer is:
haydn
'composer' is the definition.
(Haydn is an example)
'had note about unknown' is the wordplay.
'note' becomes 'n' (abbreviation**).
'about' indicates putting letters inside.
'unknown' becomes 'y' (algebra).
'had'+'n'='hadn'
'hadn' going around 'y' is 'HAYDN'.
(Other definitions for haydn that I've seen before include "Classical composer" , "Creation composer" , "Barman" , "Austrian musician" , "The Surprise symphony composer, d. 1809" .)