If you show your current attempts, they can be fixed. Have you checked the source code of the page to make sure that you are in the correct frame and select the correct element?
Generally speaking, you can clear the textbox / textarea / whatnot function from seleniums clear (), for example:
1. driver.find_element_by_id("id of textbox").clear()
or 2. driver.find_element_by_css_selector("#...").clear()
etc etc
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, , , , , -, iframe, iframe.
:
driver.switch_to_frame("frame name")
driver.find_element_by_id("textbox id").clear()
driver.switch_to_default_content()