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' indicates an anagram (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 away is 'thos'.
'a'+'p'='ap'
'ap' anagrammed 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" .)