Friend of d'Artagnan (5)
I believe the answer is:
athos
'friend of d'artagnan' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for athos that I've seen before include "Mount - musketeer" , "Appropriating" , "Literary musketeer" , "Greek monastic mountain" , "Greek mount; one of the Three Musketeers" .)