As a drug, take herb in the prescribed amount? (6)
I believe the answer is:
dosage
'the prescribed amount?' is the definition.
(I've seen this in another clue)
'as a drug take herb' is the wordplay.
'as a drug take' becomes 'do' (as in doing drugs).
'herb' becomes 'sage' (sage is a kind of herb).
'do'+'sage'='DOSAGE'
'in' is the link.
(Other definitions for dosage that I've seen before include "precise quantity" , "Provision of medicine in prescribed amounts" , "The giving of medication in prescribed amount" , "Giving of medicine in prescribed amount" , "Prescribed amount of drug" .)