Here is a very heavily clashed 3 cent nickel.
The imprint of the reverse wreath and the first I of III can be seen on the obverse (the I is seen below Liberty's cin and less prominently above and below her hair). The reverse is spectacularly clashed with a triple (at least) imprint of Liberty's profile appearing in the field and her neck in the top open area.


Basically a clash is damage to the FIELDS of a die since those are the highest points of a die. The fields are damaged when they strike the openings (recessed areas) on the opposite die and retain a ghosted image of the opposite die's outline. Then when a coin is struck it shows the ghosted image on the fields only and the clashing is not visible on the raised portions of the die.
I hope this helps!