Dumas' second character Aramis could be a swordsman (7)
I believe the answer is:
samurai
'a swordsman' is the definition.
'dumas' second character aramis could be' is the wordplay.
'dumas' second character' becomes 'u' (2nd letter of 'dumas').
'could be' indicates anagramming the letters.
'u'+'aramis'='uaramis'
'uaramis' anagrammed gives 'SAMURAI'.
(Other definitions for samurai that I've seen before include "Jap. warrior caste" , "Seven -, film directed by Kurosawa" , "Historical Japanese warrior" , "Former Japanese military caste member" , "Traditional Japanese warrior" .)