Prescription portion (4)
I believe the answer is:
dose
'prescription portion' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dose that I've seen before include "Amount of medicine to be taken at one time" , "Measure (med.)" , "some Viagra, perhaps?" , "Medicinal quantity" , "Give drugs to (a person)" .)