Knight plunged into lake to get boy (5)
I believe the answer is:
ernie
'boy' is the definition.
(I've seen this before)
'knight plunged into lake' is the wordplay.
'knight' becomes 'n' (chess).
'plunged into' indicates putting letters inside.
'lake' becomes 'erie' (Lake Erie is one of the Canadian/US 'Great Lakes').
'n' inserted within 'erie' is 'ERNIE'.
'to get' acts as a link.
(Other definitions for ernie that I've seen before include "Machine that randomly selects Premium Bond winners" , "Selector of winning premium bond numbers" , "Computer selecting Premium Bond winners" , "Premium Bond selecting machine" , "Premium?bond computer" .)