Bitter sour? That's about right (5)
I believe the answer is:
acrid
'bitter' is the definition.
(thesaurus)
'sour? that's about right' is the wordplay.
'sour?' becomes 'acid' (I've seen this before).
'that's about' indicates putting letters inside.
'right' becomes 'r' (common abbreviation).
'acid' going around 'r' is 'ACRID'.
(Other definitions for acrid that I've seen before include "Direly pungent" , "Sharp - biting" , "Bitterly pungent" , "Astringent" , "Strong and sharp" .)