Depends on what you mean; do you want it to have the same size, but with all the values set to zero...or do you want to destroy it and have an array of length 0? IN the latter case you can just have e.g. a case somewhere that lets through the array in one case and sends out an empty one in the other (wire the array through and then set the tunnel to return default if not wired for the other case...e.g.). In the former case you could just get the size of the array and initialize a new one the same size with all zeroes, or you could work with the same array and replace all elements with zeroes.
OR if the array is a control and you want to clear that you could use an invoke node to reset it to default e.g...
