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.

Thank you for correcting my mistake.I'll pay attention to it.

"I can hardly understand what she referred to" is not a correct expression to use here. What you need here is "I don't understand what she meant".