Weapon used by many a knight (8)
I believe the answer is:
lancelot
'a knight' is the definition.
(knight in Arthurian legends)
'weapon used by many' is the wordplay.
'weapon' becomes 'lance' (lance is a kind of weapon).
'used by' says to put letters next to each other.
'many' becomes 'lot' (a lot of something).
'lance'+'lot'='LANCELOT'
(Other definitions for lancelot that I've seen before include "Arthurian knight, Guinevere's lover" , "K?" , "Arthur's knight, lover of Guinevere" , "legendary knight" , "Round Table member" .)