Ireland's round lake (4)
I believe the answer is:
erie
'lake' is the definition.
(Lake Erie is one of the Canadian/US 'Great Lakes')
'ireland's round' is the wordplay.
'ireland' becomes 'eire' (poetic name for Ireland).
'round' says the letters should be written backwards.
'eire' back-to-front is 'ERIE'.
(Other definitions for erie that I've seen before include "Spooky-sounding" , "A Great American lake - sounds spooky" , "N. American lake" , "Superior companion" , "Lake between Canada and the US" .)