Great Lake with the longest shoreline (5)
I believe the answer is:
huron
'great lake with the longest shoreline' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for huron that I've seen before include "One of the Great Lakes of North America" , "Second largest of the Great Lakes" , "N. American lake" , "Great Lake" .)