Bet about northern composer (6)
I believe the answer is:
wagner
'composer' is the definition.
(Richard Wagner)
'bet about northern' is the wordplay.
'bet' becomes 'wager' (synonyms).
'about' means one lot of letters goes inside another.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'wager' going around 'n' is 'WAGNER'.
(Other definitions for wagner that I've seen before include "German composer of The Flying Dutchman" , ""The Ring of the Nibelung" composer" , "German opera composer" , "German cycle-maker" , "Richard . . . . . . composed The Flying Dutchman" .)