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' shows that the letters should be reversed in order.
'eire' reversed gives '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" .)