Doctor John was his assistant for many cases (8)
I believe the answer is:
sherlock
This clue might be a double definition.
'doctor john' is the first definition.
The answer and definition can be both people as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'his assistant for many cases' is the second definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'was' acts as a link.
Can you help me to learn more?
(Other definitions for sherlock that I've seen before include "Conan Doyle's sleuth, . . . . . . . . Holmes" , "Arthur Conan Doyle's detective" , "Fictional detective" , "Fictional sleuth . . . . . . . . Holmes" , "Mr Holmes" .)