[adj] of or relating to King Arthur and the Knights of the Round Table
ARTHURIAN is a 9 letter word that starts with A.
Webster's 1913 Dictionary
Definition:
\Ar*thu"ri*an\, a.
Of or pertaining to King Arthur or his knights. --J. R.
Symonds.
In magnitude, in interest, and as a literary origin,
the Arthurian invention dwarfs all other things in the
book. --Saintsbury.
WordNet Dictionary