Stinging bill rises and is cut unexpectedly (7)
I believe the answer is:
caustic
'stinging' is the definition.
(thesaurus)
'bill rises and is cut unexpectedly' is the wordplay.
'bill' becomes 'ac'.
'rises' says the letters should be written backwards.
'and' means one lot of letters go next to another.
'unexpectedly' is an anagram indicator.
'ac' back-to-front is 'ca'.
'is'+'cut'='iscut'
'iscut' anagrammed gives 'ustic'.
'ca'+'ustic'='CAUSTIC'
(Other definitions for caustic that I've seen before include "Bitter - corrosive" , "Harsh or corrosive" , "Chemically burning" , "Acrid" , "Scathing" .)