The quantity is the same and you add the herb to it (6)
I believe the answer is:
dosage
'the quantity' is the definition.
(dosage is a kind of quantity)
'the same and you add the herb to it' is the wordplay.
I cannot quite see how this works, but
'same' could be 'do' and 'do' is found in the answer.
'herb' could be 'sage' (sage is a kind of herb) and 'sage' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'is' is the link.
Can you help me to learn more?
(Other definitions for dosage that I've seen before include "precise quantity" , "Correct use of a prescribed drug" , "Giving of medicine in prescribed amount" , "Prescribed quantity of medicine" , "Measured amount of medicine to be taken" .)