Sharpness, sourness (7)
I believe the answer is:
acidity
I believe this clue is a double definition.
'sharpness' is the first definition.
(thesaurus)
'sourness' is the second definition.
(acidity is a kind of sourness)
(Other definitions for acidity that I've seen before include "bite" , "Corrosive quality" , "Sourness, sharpness" , "Sourness (of taste)" , "Sour quality" .)