Worsen a good accent bringing in a trace of twang (9)
I believe the answer is:
aggravate
'worsen' is the definition.
('aggravate' can be a synonym of 'worsen')
'a good accent bringing in a trace of twang' is the wordplay.
'good' becomes 'g' (abbreviation).
'accent' becomes 'grave' (grave is a kind of accent**).
'bringing in' is an insertion indicator.
'trace of' says to take the initial letters.
The initial letter of 'twang' is 't'.
'a'+'t'='at'
'grave' going around 'at' is 'gravate'.
'a'+'g'+'gravate'='AGGRAVATE'
(Other definitions for aggravate that I've seen before include "Exasperate or exacerbate" , "Make more serious" , "Make a bad situation worse" , "Make (a problem) worse" , "Annoy; make worse" .)