Here's why:
* Short vowel sounds are typically quick and sharp. In "clap," the "a" sound is quick and doesn't linger.
* Long vowel sounds are typically drawn out and can be represented by a single vowel letter with a silent "e" at the end of the word, or a double vowel.
Since "clap" has a single vowel letter and no silent "e" at the end, we know it's the short vowel sound.