Crossword Genius

Make a deduction (8)

Ross

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5

8

subtract

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'make' is the definition.
The definition and answer can be both related to thinking as well as being verbs in their base form.
Maybe they are linked in a way I don't understand?

I don't understand the rest of the clue.

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(Other definitions for subtract that I've seen before include "Withdraw" , "Take an amount away" , "Deduct" , "dock" , "Take away (a number)" .)

I've seen this clue in The Globe and Mail.
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