Defending the crack block

I couldn't say it any better then Bill just did. Be physical with that WR who is cracking. The corner needs to be aggressive against the run and "fill the crack", coming up to replace. A second way of getting through the crack is for the OLB or SS to read the play quickly and aggressively step up to defend the run so the angle for the crack is gone and there is nothing to hit but their back. A third way is to use a speed rip past the crack. Have the OLB or SS run right at the crack and perform a rip move with the shoulder closer to the LOS while sprinting past the cracker. I mentioned the last option last because I prefer the other two.
Oh, and CUI, it sounds like you're playing the same type of team I am with the same defensive scheme I am. Good luck.