Sadness of a soft-headed musketeer (6)
I believe the answer is:
pathos
'sadness' is the definition.
(thesaurus)
'of a soft-headed musketeer' is the wordplay.
'of' is an anagram indicator (the answer comes from or 'of' the letters).
'soft' becomes 'p' (musical abbreviation for piano, 'soft').
'headed' means to remove the first letter (head can mean to remove the top from something).
'musketeer' becomes 'Athos' (musketeer from The Three Musketeers).
'athos' with its first letter taken off is 'thos'.
'a'+'p'='ap'
'ap' with letters rearranged gives 'pa'.
'pa'+'thos'='PATHOS'
(Other definitions for pathos that I've seen before include "Something evoking pity" , "feeling of pity" , "Quality inspiring pity" , "Quality that arouses pity or sadness" , "A quality that arouses pity or sorrow" .)