Medicate deer around south (4)
I believe the answer is:
dose
'medicate' is the definition.
(I know that dose is a kind of medicate)
'deer around south' is the wordplay.
'deer' becomes 'doe' (female deer).
'around' means one lot of letters goes inside another.
'south' becomes 's' (abbreviation).
'doe' going around 's' is 'DOSE'.
(Other definitions for dose that I've seen before include "Measure of medicine" , "some Viagra, perhaps?" , "Medicinal quantity" , "Medicate" , "Measure (med.)" .)