Her breath is bad! (6)
I believe the answer is:
bertha
'her' is the definition.
'breath is bad' is the wordplay.
'is bad' indicates anagramming the letters (I've seen 'being bad' mean this).
'breath' anagrammed gives 'BERTHA'.
(Other definitions for bertha that I've seen before include "big woman?" , "collar" , "43-ton German howitzer, Big _____" , "Girl's name - big German WWI gun" , "Breath (anag) - girl's name" .)