Swear about American soprano (4)
I believe the answer is:
cuss
'swear' is the definition.
(I know that swear can be written as cuss)
'about american soprano' is the wordplay.
'about' becomes 'c' (abbreviation for circa).
'american' becomes 'us' (abbreviation for United States).
'soprano' becomes 's' (musical abbreviation).
'c'+'us'+'s'='CUSS'
(Other definitions for cuss that I've seen before include "Stubborn person" , "Use bad language (informal)" , "Swear (slang)" , "Oath; (annoying sort of) person" , "Blaspheme" .)