What Salem's Lot horrifically has — the ultimate in scary foolishness (9)
I believe the answer is:
asininity
'foolishness' is the definition.
(I know that asininity is a type of foolishness)
'what salem's lot horrifically has the ultimate in scary' is the wordplay.
I cannot really see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
'in' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for asininity that I've seen before include "Stupidity" , "Extreme foolishness" , "Foolishness as of donkey" , "Idiocy" .)