Arthurian knight taking his weapon back to lake (8)
I believe the answer is:
lancelot
'arthurian knight' is the definition.
(knight in Arthurian legends)
'weapon back to lake' is the wordplay.
'weapon' becomes 'lance' (lance is a kind of weapon).
'back' is a reversal indicator.
'lake' becomes 'l' (geographical abbreviation).
'to'+'l'='tol'
'tol' back-to-front is 'lot'.
'lance'+'lot'='LANCELOT'
'taking his' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
(Other definitions for lancelot that I've seen before include "queen's lover" , "K?" , "Knight of the Round Table and close friend of King Arthur's wife, Guinevere" , "One of King Arthur's knights, lover of Guinevere" , "A Knight of the Round Table, lover of Guinevere (8)." .)