A Canadian lake - before one plunged in (4)
I believe the answer is:
erie
'a canadian lake' is the definition.
'erie' can be an answer for 'lake' (Lake Erie is one of the Canadian/US 'Great Lakes'). I'm not sure about the 'a canadian' bit.
'before one plunged in' is the wordplay.
'before' becomes 'ere' (poetic term for 'before').
'one plunged' becomes 'i' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'ere' enclosing 'i' is 'ERIE'.
Can you help me to learn more?
(Other definitions for erie that I've seen before include "Lake between Canada and the US" , "The lake sounds weird" , "N. American lake" , "Superior companion" , "Buffalo's lake" .)