Does vary the amount to be taken (4)

I believe the answer is:

dose

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'amount to be taken' is the definition.
I can't tell whether this definition defines the answer.

'does vary' is the wordplay.
'vary' is an anagram indicator.
'does' anagrammed gives 'DOSE'.

'the' is the link.

Can you help me to learn more?

(Other definitions for dose that I've seen before include "Give drugs to (a person)" , "Shot of medicine" , "Medicinal amount" , "Helping of medicine" , "Measured amount of medicine" .)

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

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