Behold, through window, legendary knight ... (8)
I believe the answer is:
lancelot
'legendary knight' is the definition.
(I've seen this before)
'behold through window' is the wordplay.
'behold' becomes 'lo'.
'through' means one lot of letters goes inside another (I've seen this in other clues).
'window' becomes 'lancet' (lancet window is a kind of window).
'lo' going into 'lancet' is 'LANCELOT'.
(Other definitions for lancelot that I've seen before include "legendary knight" , "Most famous of King Arthur's knights, lover of Guinevere" , "Arthurian knight, Guinevere's lover" , "Round Table knight who loved Guinevere" , "'Knight of the round table, Guinevere's lover (8)'" .)