Better solution I've just founded when you can't cut because of many cords :

• Close your patch from pure data

• Open the patch.pd with a text editor like notepad ++

• Find the line where your canvas is defined
  should look like it : #X obj 154 346 cnv 15 500 100 empty empty mon_petit_canvas 20 12 0

• Cut this line and past it after the first line of the text file.

• Save and Close in text editor

Voilà !