Please find the instructions mentioned below:
Method 1:
Go to Products > Select Product > Show HTML
Remove unwanted DIVs, HTML and Code to Clear The Formatting
Usually description copied from other store also copies the formatting code which needs to be removed to show an appropriate layout.
Method 2:
Copy all content from product description show HTML and paste it on a notepad and copy it again from notepad and paste it to text editor. This also clears formatting.