One "Thomas", a musketeer (5)

I believe the answer is:

athos

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'musketeer' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for athos that I've seen before include "Comrade of Porthos and Aramis" , "Brave soldier" , "Literary musketeer" , "Greece's holy mount" , "Appropriating" .)

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