Boy from north in lake (5)
I believe the answer is:
ernie
'boy' is the definition.
(boy's name)
'north in lake' is the wordplay.
'north' becomes 'N' (abbreviation).
'in' means one lot of letters goes inside another.
'lake' becomes 'erie' (Lake Erie is one of the Canadian/US 'Great Lakes').
'n' inserted into 'erie' is 'ERNIE'.
'from' is the link.
(Other definitions for ernie that I've seen before include "Eric's TV partner" , "Premium Bond picker" , "Sergeant Bilko, for one" , "Premium Bond selecting machine" , "Els, for example" .)