He dispensed treatment in Post Office Boxes (11)
I believe the answer is:
hippocrates
'he dispensed treatment' is the definition.
(ancient Greek doctor)
'in post office boxes' is the wordplay.
'in' becomes 'hip' (both can mean fashionable).
'post office' becomes 'po'.
'boxes' becomes 'crates' (crate is a kind of box).
'hip'+'po'+'crates'='HIPPOCRATES'
(Other definitions for hippocrates that I've seen before include "He offered oath" , "he produced an oath" , "Greek physician" , "celebrated doctor" .)