Bet about new composer (6)
I believe the answer is:
wagner
'composer' is the definition.
(Richard Wagner)
'bet about new' is the wordplay.
'bet' becomes 'wager' (synonyms).
'about' indicates putting letters inside.
'new' becomes 'n' (abbreviation as in New Testament NT).
'wager' going around 'n' is 'WAGNER'.
(Other definitions for wagner that I've seen before include "German operatic composer, d. 1883" , "German composer of The Ring Cycle" , ""The Ring of the Nibelung" composer" , "German opera composer" , "Richard . . . . . . composed The Flying Dutchman" .)