Physician's nickname (3)
I believe the answer is:
doc
'physician's nickname' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for doc that I've seen before include "Physician (colloq.)" , "Holliday (in 17 24)" , "Medical practitioner (abbr)" , "bones" , "Disney dwarf" .)