A sharp pain — darn! (6)
I believe the answer is:
astute
'a sharp' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for astute that I've seen before include "on the ball" , "Shrewd and clever" , "smart (with some self-interest?)" , "Subtle" , "Having hardheaded intelligence" .)