Twang with fat, round backing (5)
I believe the answer is:
drawl
'twang' is the definition.
(thesaurus)
'with fat round backing' is the wordplay.
'with' becomes 'w' (abbreviation**).
'fat' becomes 'lard' (I've seen this before).
'round' indicates putting letters inside.
'backing' says the letters should be written backwards.
'w' placed into 'lard' is 'lward'.
'lward' back-to-front is 'DRAWL'.
(Other definitions for drawl that I've seen before include "Speak with lengthy vowel sounds" , "Speak lazily with long vowel sounds" , "Speak slowly with long vowels sounds" , "Jimmy Stewart spoke with one" , "Way of speaking slowly" .)