Weapon left by old-time knight (8)
I believe the answer is:
lancelot
'knight' is the definition.
(knight in Arthurian legends)
'weapon left by old-time' is the wordplay.
'weapon' becomes 'lance' (lance is a kind of weapon).
'left by' becomes 'l' (I've seen this in other clues).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'time' becomes 't' (abbreviation).
'lance'+'l'+'o'+'t'='LANCELOT'
(Other definitions for lancelot that I've seen before include "queen's lover" , "legendary knight" , "'Knight of the round table, Guinevere's lover (8)'" , "One of King Arthur's knights, lover of Guinevere" , "Hero" .)