have it in (one)
To have or possess a specific skill or trait. Wow, Alice is a beautiful singer—I didn't know she had it in her. Honestly, I'm not sure he has it in him to deal with this.
have it in one
Have the ability to accomplish something. As Sir Arthur Conan Doyle put it in A Study in Scarlet (1887), "I know well that I have it in me to make my name famous."