senselessness ... however I like asslessness better. It has 11 letters and is possibly an actual word. As in "Those chaps display an awful lot of asslessness"
In the absence of a definition, I'll provide one. What is a 13 letter word for vertical? Answer: perpendicular
13 letters in the word "Scarlet Letter"
The longest word that can be made using these letters is hydrolytic.
You can't spell any 13 letter words from just 12 letters. You need the final S to spell "procrastinate."
The letters do not spell any 13 letter word. However they can be used to spell the 9 letter word instincts.