All you need to do is either cycle through the textbox  controls with a "For
Next" loop or list them all individually and set the .Value property to
nothing

textboxcontrolname.Value = "" 'substitute the actual name of you control
for textboxcontrolname

You may want to set up a UserForm_Initialize event for you form  that will
clear all the textbox controls before the user sees the form. Then you may
find that an Ok and a Cancel button  are all you need and not a "Clear"
button.