Is it possible to have Live form filling for completion of details in a contract without having to take the file back to Word and clean up the document?

There are two possibilities depending on how that data is being modified....

You can "Replace" a document in an envelope using the "Correct" envelope feature. This basically removes the old document and replaces with a new document. However, if an action has been taken on the envelope such as a Recipient signing then you may not be able to "correct" the envelope because it is considered completed or in process. This would cover a scenario where the text must be edited in Word or other text editor.

If you add a DocuSign field, let's say a Text field for the "Customer Name" and you set that field to read-only but after you send the envelope you notice it is spelled incorrectly. You again can "correct" the field contents as long as no action has been taken by a Recipient.

Strictly speaking if you upload a 500 word document, and you only added one field for Signature, you cannot go to the DocuSign envelope and edit the text directly.