Probably an acute... Fungal infection causing the crack in the skin. This is one of the presentations of this problem. The best way (in my opinion) is to use a cream that kills the fungus, instead of using one that just slows down the replication. Lotrimin ultra, or Lamisil (terbinafine) creams are easy to find and work the vast majority of the time.
Fungal issue. Usually the cracking underneath the toes are due to fungus. Fungal creams will help. If there is not fungus then try to leave it alone as much as possible to let it heal by itself. If it does not heal see your podiatrist.