Neither flat nor blunt (5)
I believe the answer is:
sharp
'blunt' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sharp that I've seen before include "Acute; pointed" , "Cutting; note" , "Fly - biting" , "Piercing; clearly defined" , "factor in scoring?" .)