What do you mean by hash clash?

Hashing is not perfect. Occasionally, a collision occurs when two different keys hash into the same hash value and are assigned to the same array element. Programmers have come up with various techniques for dealing with this conflict. A common way to deal with a collision is to make a linked list of entries that have the similar hash value. For example, let the key of every entry hashes to the similar hash value and this result in both being assigned to the similar array element of the hashtable.