Here's why:
* The "a" sound is short: The "a" in "clasp" has a short, "a" sound, like in the word "cat".
* No other vowels are stressed: The other vowels in "clasp" (the "i" in "clasp" is silent) are not stressed, so they don't have long sounds either.
Therefore, "clasp" is pronounced with a short vowel sound and no long sounds.