This is an example of onomatopoeia, the formation of a word relates to the sound it makes. These words are used in the literary world to make reading a story more interesting. You can imagine the sound in the story. Words that have the 'cl' sound at the beginning relate to things that collide and the sound it would make. For example, 'I heard a clatter in the kitchen'. This tells me what sound I heard. Another example which is about air include words beginning with 'wh' - whoosh, whisper, whiff, whizz, whip.

I'm not a linguist but I'm pretty sure that 'cla' would be called a morpheme. Most likely 'clap' derives from a longer word in an older form of English or French. This older word would have used 'cla' to give meaning to that word. This would then lead to other similar words using this morpheme.

I thought you were asking about the 'cla'