Bet involving north German composer (6)
I believe the answer is:
wagner
'german composer' is the definition.
(I know that Wagner is a composer)
'bet involving north' is the wordplay.
'bet' becomes 'wager' (synonyms).
'involving' indicates putting letters inside.
'north' becomes 'N' (abbreviation).
'wager' enclosing 'n' is 'WAGNER'.
(Other definitions for wagner that I've seen before include "German opera composer" , "Composer honoured in Bayreuth" , "German composer of The Ring Cycle" , "We rang up the composer" , "Richard . . . . . . composer of 'The Flying Dutchman' and 'Lohengrin'" .)