Founder's pen doesn't show how to beat budget (10)

I believe the answer is:

underspend

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

'to beat budget' is the definition.
I can't tell whether this definition defines the answer.

'founder's pen doesn't show' is the wordplay.
'show' indicates the answer is hidden within the clue.
'UNDERSPEND' can be found hidden inside 'founders pen doesnt'.

'how' acts as a link.

Can you help me to learn more?

(Another definition for underspend that I've seen is " Pay out too little".)

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

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