One "Thomas," a musketeer (5)
I believe the answer is:
athos
'musketeer' is the definition.
(musketeer from The Three Musketeers)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for athos that I've seen before include "Comrade of Porthos and Aramis" , "Brave soldier" , "Literary musketeer" , "Greece's holy mount" , "Appropriating" .)