Eldest of the Three Musketeers (5)
I believe the answer is:
athos
'eldest of the three musketeers' 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 "Brave soldier" , "Literary musketeer" , "Dumas musketeer" , "Greek mount; one of the Three Musketeers" , "Mount - musketeer" .)