Detective stifled by a town's bitterness (7)
I believe the answer is:
acidity
'bitterness' is the definition.
(thesaurus)
'detective stifled by a town's' is the wordplay.
'detective' becomes 'di' (Detective Inspector abbreviation).
'stifled by' means one lot of letters goes inside another (I've seen this in other clues).
'town' becomes 'city' (I've seen this in other clues).
'a'+'city'='acity'
'di' inserted inside 'acity' is 'ACIDITY'.
(Other definitions for acidity that I've seen before include "bite" , "Sour taste" , "Corrosive quality" , "Tartness" , "Tart quality" .)