Knight, behold, impaling one with double-edged blade (8)
I believe the answer is:
lancelot
'knight' is the definition.
(knight in Arthurian legends)
'with double-edged blade' is the wordplay.
'with' indicates putting letters inside.
'double edged' becomes 'lo' (I can't justify this - if you can you should believe this answer much more).
'blade' becomes 'lancet' (I've seen this before).
'lo' going within 'lancet' is 'LANCELOT'.
'behold impaling one' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for lancelot that I've seen before include "Arthurian knight, Guinevere's lover" , "Hero" , "Arthur's knight, lover of Guinevere" , "Most famous of King Arthur's knights, lover of Guinevere" , "legendary knight" .)