Duff up tiny bloke, initially earn more cash if you do this! (4,8)

I believe the answer is:

work overtime

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

'tiny bloke initially earn more cash if you do this' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'duff up' is the wordplay.
I cannot really see how this works, but
'up' could be 'over' and 'over' is found within the answer.
The remaining letters 'worktime' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Independent.

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