Buck's companion swallows small prescription from the chemist (4)
I believe the answer is:
dose
'prescription from the chemist' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'buck's companion swallows small' is the wordplay.
'buck's companion' becomes 'doe' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'swallows' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'doe' going around 's' is 'DOSE'.
Can you help me to learn more?
(Other definitions for dose that I've seen before include "Medicinal quantity" , "Shot of medicine" , "Measure (med.)" , "Helping of medicine" , "Prescribed amount of medicine" .)