A sheep is in The Three Musketeers (6)
I believe the answer is:
aramis
'the three musketeers' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'a sheep is' is the wordplay.
'sheep' becomes 'ram' (ram is a kind of sheep).
'a'+'ram'+'is'='ARAMIS'
'in' is the link.
Can you help me to learn more?
(Other definitions for aramis that I've seen before include "One of Dumas's Musketeers" , "One Musketeer" , "Fictional musketeer" , "One of Dumas' three musketeers" , "Literary musketeer" .)