Billions, when you shouldn't have enough! (5)

I believe the answer is:

basta

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'enough' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'billions when you shouldn't have' is the wordplay.
'billions' becomes 'b' (abbreviation).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'you shouldn't have' becomes 'ta' (both can mean 'thank you').
'b'+'as'+'ta'='BASTA'

Can you help me to learn more?

I've seen this clue in The Independent.

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