Doctor has to sound like a duck (5)
I believe the answer is:
quack
I believe this is a double definition.
'doctor' is the first definition.
(I know that quack is a type of doctor)
'to sound like a duck' is the second definition.
(I've seen this before)
'has' is the link.
(Other definitions for quack that I've seen before include "Sound like a duck" , "Duck's cry or dud doctor" , "Charlatan - cry of duck" , "phoney" , "Unqualified man" .)