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' means one lot of letters go next to another.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'hour' anagrammed 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" .)