Doctor's first with old herb — the prescribed amount (6)
I believe the answer is:
dosage
'prescribed amount' is the definition.
(I've seen this in another clue)
'doctor's first with old herb' is the wordplay.
'doctor's first' becomes 'd' (sst letter of 'doctor').
'with' says to put letters next to each other.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'herb' becomes 'sage' (sage is a kind of herb).
'd'+'o'+'sage'='DOSAGE'
'the' acts as a link.
(Other definitions for dosage that I've seen before include "Quantity of medication" , "measure" , "Provision of medicine in prescribed amounts" , "Prescribed amount of drug" , "quantity of drug" .)