One of the musketeers muffed a shot (5)
I believe the answer is:
athos
'one of the musketeers' is the definition.
(I've seen this in another clue)
'muffed a shot' is the wordplay.
'muffed' is an anagram indicator (letters muffed or messed up).
'a'+'shot'='ashot'
'ashot' is an anagram of 'ATHOS'.
(Other definitions for athos that I've seen before include "Colleague of Porthos, Aramis" , "Dumas musketeer" , "Mount - musketeer" , "Literary musketeer" , "Greek mount; one of the Three Musketeers" .)