Doctor has old bovine sound (3)
I believe the answer is:
moo
'bovine sound' is the definition.
(I've seen this in another clue)
'doctor has old' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'has' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'mo' enclosing 'o' is 'MOO'.
(Other definitions for moo that I've seen before include "Bovine low" , "Guernsey noise?" , "Cow's sound" , "Irritating woman" , "Low (of cattle)" .)