It's hard, but makes for greater incisiveness (5)
I believe the answer is:
steel
'it's hard' is the definition.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for steel that I've seen before include "Form of iron" , "Strength, determination" , "Hard, strong, grey alloy" , "Make hard and unfeeling" , "May be cold" .)