* The sound of clapping: This is a sharp, distinct sound, usually short in duration. So, in this sense, yes, a clap is a short sound.
* The act of clapping: This is an action, and its duration can vary. A quick clap is short, but sustained applause involves multiple, repeated claps, making it longer.
Therefore, the answer depends on what you're referring to: the sound itself or the action of clapping.