Smashing hour by northern lake (5)
I believe the answer is:
huron
'lake' is the definition.
(I've seen this before)
'smashing hour by northern' is the wordplay.
'smashing' indicates an anagram.
'by' is a charade indicator (letters next to each other).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'hour' with letters rearranged gives 'huro'.
'huro'+'n'='HURON'
(Other definitions for huron that I've seen before include "N. American lake" , "Great Lake" , "One of the Great Lakes of North America" , "Second largest of the Great Lakes" .)