Does vary the amount taken (4)
I believe the answer is:
dose
'amount taken' is the definition.
I can't judge whether this definition defines the answer.
'does vary' is the wordplay.
'vary' indicates anagramming the letters.
'does' is an anagram of 'DOSE'.
'the' is the link.
Can you help me to learn more?
(Other definitions for dose that I've seen before include "Medicinal amount" , "Shot of medicine" , "Measured quantity of medicine" , "Prescribed amount of medicine" , "Measured amount of medicine" .)