Game changer with young guy making lots of money (8)

'game changer' is the definition.
The definition suggests a singular noun which matches the answer.

'young guy making lots of money' is the wordplay.
'young guy making' becomes 'mega' (I can't explain this - if you can you should give a lot more credence to this answer).
'lots of' means to remove the last letter (most but not all of the letters).
'money' becomes 'bucks' (informal term for dollars).
'bucks' with its last letter taken off is 'buck'.
'mega'+'buck'='MEGABUCK'

'with' is the link.

Can you help me to learn more?