AHK, 10 bytes (Windows)

clipboard= 

Clears contents of the clipboard

AHK, 10 bytes (Windows)

clipboard= 

Clears text content of the clipboard. If clipboardAll is used it clears everything.

From the docs -

Clipboard is a built-in variable that reflects the current contents of the Windows clipboard if those contents can be expressed as text. By contrast, ClipboardAll contains everything on the clipboard, such as pictures and formatting.