Wise knight entering lake (5)
I believe the answer is:
ernie
'wise' is the definition.
(comedian Ernie Wise)
'knight entering lake' is the wordplay.
'knight' becomes 'N' (chess abbreviation).
'entering' means one lot of letters goes inside another.
'lake' becomes 'erie' (Lake Erie is one of the Canadian/US 'Great Lakes').
'n' going inside 'erie' is 'ERNIE'.
(Other definitions for ernie that I've seen before include "One regularly drawing" , "Eric's other half" , "Premium Bond picker" , "Machine that randomly selects Premium Bond winners" , "Boy" .)