Words on an arrow (3-3)
I believe the answer is:
one-way
'words on an arrow' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for one-way that I've seen before include "Like non-return valve" , "This street is single-directional" , "Moving in a single direction only" , "Allowing travel in one direction only" , "Moving in a sole direction" .)