How is tightwad using convenience? (8,1,5)

I believe the answer is:

spending a penny

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'how is tightwad' is the definition.
I can't judge whether this defines the answer.

'using convenience?' is the wordplay.
'using' becomes 'spending' (spending is a kind of using).
'convenience?' becomes 'apenny' (I can't explain this - if you can you should believe this answer much more).
'spending'+'apenny'='SPENDING A PENNY'

Can you help me to learn more?

(Another definition for spending a penny that I've seen is " Visiting gents".)

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

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