I have several questions:
Which OS are you using?
Are you trying to insert a clipping into a message editor?
How is the clipping being inserted -- is it from the Clippings context menu, or from the keyboard paste dialog (invoked in Windows 10 by pressing Alt+Shift+Y, or Cmd+Option+V on macOS)?
Is the email message plain text or HTML formatted? Are there any HTML tags in the clipping being pasted?
Is this issue consistently happening with any clipping, and in any message editor window (composing a new message or replying to a message)?