Swore badly, having deteriorated (5)
I believe the answer is:
worse
'having deteriorated' is the definition.
(I've seen this in another clue)
'swore badly' is the wordplay.
'badly' indicates anagramming the letters.
'swore' with letters rearranged gives 'WORSE'.
(Other definitions for worse that I've seen before include "Less satisfactory" , "Opposite of better" , "Of poorer quality" , "More awful or unwell" , "More serious" .)