Betting morning? What rubbish! (4)

I believe the answer is:

spam

I'm a little stuck... Click here to teach me more about this clue!

'what rubbish' is the definition.
I can't tell whether this definition defines the answer.

'betting morning?' is the wordplay.
'betting' becomes 'sp'.
'morning?' becomes 'am'.
'sp'+'am'='SPAM'

Can you help me to learn more?

(Other definitions for spam that I've seen before include "Unwelcome approach" , "Send unsolicited mail" , "email messages" , "Computer junk mail" , "Unwanted communication" .)

I've seen this clue in the Sydney Morning Herald.

Want a hint initially instead of a full solution? Install my app