Pennsylvania's Gem City (4)
I believe the answer is:
erie
'pennsylvania's gem city' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for erie that I've seen before include "North american lake sounds spooky" , "Spooky-sounding" , "N. American lake" , "Great lake in Eire" , "One of the Great Lakes" .)