I think it would depend on WHY you got the P. Were you a rockstar clinically but did poorly on the shelf? That's probably the easiest thing to overcome since your MSPE and your LORs will say awesome things about you; with good Step scores you'd probably be fine.
It also depends on your school's distribution. If you're at a school where only 10% get honors and the rest get pass, then nobody knows whether you were in the 89th or 9th percentile of that group. On the other hand, if 67% got honors, 12% got high pass, and you were in the 21% who only passed, then that looks much worse. Even then it still matters what the reason was.
At this point, do whatever you can to be a rockstar for the rest of your clerkships and SubIs. A lone P in a sea of Honors probably won't matter much in the long run; a sea of passes in a school where most everyone gets honors, well, that's another story.