Spend the summer dividing 1,000 by 200 in a rough calculation (8)

I believe the answer is:

estivate

'spend the summer' is the definition.
(estivation is the summer equivalent of hibernation)

'dividing 1000 by 200 in a rough calculation' is the wordplay.
I cannot really see how this works, but
'by' could be 'at' and 'at' is located in the answer.
This explanation may well be incorrect...

I've seen this clue in The Guardian.

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