Anonymous person taking small quantity of medication (4)
I believe the answer is:
dose
'medication' is the definition.
(dose is a kind of medication)
'anonymous person taking small quantity' is the wordplay.
'anonymous person' becomes 'do' (I can't justify this - if you can you should believe this answer much more).
'taking' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'quantity' becomes 'e' (e is a kind of quantity).
'do'+'s'+'e'='DOSE'
'of' is the link.
Can you help me to learn more?
(Other definitions for dose that I've seen before include "Measure (med.)" , "Measure of medicine" , "Helping of medicine" , "Measured quantity of medicine" , "Medicinal amount" .)