Psst ...
A: This is the fear of horses
Q: What is hippophobia?
or
A: Hippophobia is the fear of this.
Q: What are horses?
or some other similair structure--but "Tell me what hippophobia is" seems like you want "the fear of horses", not "what is the fear of horses?"
Just chiming in
Stephen
PS. I hope I'm not the only one who just assumed it would be the fear of hippopotamuses!