An effectively directional missile (5)

I believe the answer is:

arrow

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'missile' is the definition.
(arrow is a kind of missile)

I don't understand the remainder of the clue.

(Other definitions for arrow that I've seen before include "Arm" , "Missile, bolt" , "Early guided missile" , "The bowman's missile" , "Missile fired from bow" .)

I've seen this clue in the Evening Standard.

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