I have a Nano (clone) that I am powering from a regulated supply via the 5V pin; I have checked this at the board and my multimeter reads 4.92V, which seems close enough (?). The main reason I don't use the microUSB jack is a space constraint. In any case, thus far it does work fine.
Except the leftmost (considered with the microUSB jack as "top") of the four onboard LEDs, on the opposite side from the rx/tx lights, blinks at a steady one second interval. Searching around, the only association I've found for that LED is that it is connected to D13, which I'm not using for anything.
What does this indicate? I have not tried disabling it (by setting 13 low) but I am hoping to do so if it is a benign issue.