Keystone State eponym (4)
I believe the answer is:
penn
'keystone state eponym' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for penn that I've seen before include "US state (abbrev.)" , "Colonising Quaker" , "Early Quaker leader" , "William ---, Quaker colonist" , "founder of colony" .)