Theory:

Spaulding: Can I help you?

Holmes: Yes. How can I get to the Strand?

Spaulding: Third on the right, and fourth on the left. (He goes back into the shop and closes the door.)
 
Holmes: He’s a clever young man, Watson.

Watson: Is he? Why did you ask about the Strand? You know London very well! You wanted to see him.

Holmes: Did you notice his trousers?

Watson: His trousers? No. But I saw you hit the ground with your stick?

Holmes: My dear Watson, this is not the time for discussion. I must go and look at the road behind Saxe-Coburg Square. You had to go to work.
 
Watson: Yes, to the hospital.

Holmes: Right, but I want your help tonight. Come at ten o’clock
Explanation:
 
Spaulding asked if he could help Mr. Holmes? In response, Holmes instead if interrogating Spaulding he asked the directions to the Shore. Spaulding told Holmes has to go third on the right, and fourth on the left to go the Shore. After giving directions to Holmes, immediately Spaulding goes inside the shop and locks the door. Holmes said to Watson that Spaulding was a smart young man.
 
Watson asked Holmes is Spaulding really smart and even asked him why he wants the directions to the Strand as he knew London thoroughly. Do you want to see how Spaulding look and is that the reason to knock the door and ask him the directions? Yes, the reason behind this is Holmes wanted to see the appearance of the Spaulding.
 
Holmes asked if Watson has noticed how the Spaulding's pants look. Watson replied that he didn't observe the trousers of Spaulding and asked why his trousers, but he said he observed Holmes tapping the ground with his stick. Holmes said he wants to go and see the road behind Saxe-Coburg Square and it is not the time for answering his questions.
 
Holmes asked if Watson had to go to the job. In response, Watson replies he has to go to the hospital. Holmes told Watson could proceed, but he wants his help that night and asked him to come at ten O'clock.
 
Meanings of difficult words:
  
Numbers
Word
Meanings
1.
StrandA road in London.
2.
TrousersPants.
3.
DiscussionTalk.
4.
Notice Observe.
Reference:
State Council of Educational Research and Training (2019). Term-1 English Standard-7. The Red-Headed League by Sir Arthur Conan Doyle (pp. 45-49). Published by the Tamil Nadu Textbook and Educational Services Corporation.