An effectively directional missile (5)
I believe the answer is:
'missile' is the definition.
(arrow is a kind of missile)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for arrow that I've seen before include "Arm" , "Missile, bolt" , "Early guided missile" , "The bowman's missile" , "Missile fired from bow" .)