Bet on songs being out-of-date (4,7)
I believe the answer is:
back numbers
'bet' is the definition.
I know nothing about this answer so I can't tell whether this works.
'on songs being out-of-date' is the wordplay.
'on' means one lot of letters go next to another.
'songs' becomes 'numbers' (as in musical numbers).
'being out-of-date' becomes 'back' ('back' can be a synonym of 'out-of-date').
'numbers' after 'back' is 'BACK NUMBERS'.
Can you help me to learn more?
(Other definitions for back numbers that I've seen before include "old hat!" , "Epidurals" .)