The correct answer is: despite the heat, men still dressed up.
The auhtor is describing a very hot weather, he does some examples so the readers can imagine it. By mentioning that men used "stiff collars" he shows us that thay are dressed up probably for work. The stiff collar shirts are used in as a formal clothes in our society. By mentioning the time of the day "nine in the morning" it is more obvious that they are going to work, that is the reason that despite the weather they are forced to use those types of clothing.
By using the word "wilted" we can imagine the heat those men were feeling as it is used to describe the process a plant suffers when it is exposed to heat, they dehydrate and become limp and drooping. With the men's sweat the stiff collars are no more stiff.
