Captain's doctor (4)
I believe the answer is:
cook
I believe this is a double definition.
'captain's' is the first definition.
(captain james cook)
'doctor' is the second definition.
(thesaurus)
(Other definitions for cook that I've seen before include "Captain of HMS Endeavour" , "Heat food" , "Kitchen worker" , "18th-century explorer" , "Prepare food" .)