Deer swallowing small amounts of medicine (5)
I believe the answer is:
doses
'amounts of medicine' is the definition.
(a dose is a prescribed amount of medicine)
'deer swallowing small' is the wordplay.
'deer' becomes 'does' (I've seen this in other clues).
'swallowing' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'does' placed around 's' is 'DOSES'.
(Other definitions for doses that I've seen before include "Quanitites of medicine" , "Specific quantities" , "Potion portions" , "Prescribed quantities of medicine" , "Medicates" .)