Based on paragraph 9 of The Fountainhead, a "crusader" is someone who does what?
I believe that you asking about this line of text:
Your professor of structural engineering acted quite the crusader on your behalf.
It's not paragraph 9, nor is it page 9 of my text, but it is early in the book, which is why I think that you are referring to this line.
Howard Roark is speaking to the Dean at the architecture school that Roark as just been expelled from. Roark was expelled for making his designs overly modern. Roark, throughout the novel, designs what he wants to design. He does not care to follow the conventions of what other architects say is the "right way" to design. A few of Roark's professor's see Roark's brilliance. His professor of structural engineering recognizes that while Roark's designs might look unconventional, they are structurally brilliant. He acts like a "crusader" for Roark, because the decision to expel Roark was decided by a committee. There were members of the committee that wanted to see Roark gone, and other members that wanted to see Roark stay. The professor of structural engineering defended Roark's designs and spoke in favor of Roark staying at the school. That's what "crusader" is referring to here. A defender.