The line is "I'm afraid it's a case of the warhorse and the drum."
00:11:24 Dr. Clarkson: It's kind of you to take an interest.
00:11:27 Mrs. Crawley: I'm afraid it's a case of the warhorse and the drum.
00:11:31 Mrs. Crawley: You know my late husband was a doctor.
00:11:33 Dr. Clarkson: I do. I'm familiar with Dr. Crawley's work.
It means, "A warhorse will always answer to the war drum for the rest of his/her life."
Mrs. Crawley wants to say that as a doctor's wife she is and always will be interested in patients and hospitals.