Hydromagnetic.
oystercatcher
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
Correspondent
13 letters in the word "Scarlet Letter"
The longest word that can be made using these letters is hydrolytic.
stereotypical
Hydromagnetic.
advertisementaccompanimentacupuncturistapportionment
mistletoxmasx
no. no.
Presbyterians
yellowjackets
You can't spell any 13 letter words from just 12 letters. You need the final S to spell "procrastinate."
13
The letters do not spell any 13 letter word. However they can be used to spell the 9 letter word instincts.