A sheep is a Dumas character (6)
I believe the answer is:
aramis
'a dumas character' is the definition.
(I've seen this in another clue)
'a sheep is' is the wordplay.
'sheep' becomes 'ram' (male sheep).
'a'+'ram'+'is'='ARAMIS'
(Other definitions for aramis that I've seen before include "One of Dumas' three musketeers" , "Fictional musketeer" , "Literary musketeer" , "One Musketeer" , "One of Dumas's Musketeers" .)