A bad shot for a musketeer (5)
I believe the answer is:
'musketeer' is the definition.
(I know this)
'a bad shot for a' is the wordplay.
'a bad' indicates an anagram.
'for' says to put letters next to each other (I've seen this in other clues).
'shot' anagrammed gives 'thos'.
'thos' put after 'a' is 'ATHOS'.
(Other definitions for athos that I've seen before include "Dumas musketeer" , "Appropriating" , "Greek mount; one of the Three Musketeers" , "Greece's holy mount" , "Colleague of Porthos, Aramis" .)