Musketeer has a pitiable air, having lost hat (5)

I believe the answer is:

athos

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'musketeer' is the definition.
(musketeer from The Three Musketeers)

I don't understand the rest of the clue.

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

I've seen this clue in The Irish Times.

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