Unidirectional (3-3)
I believe the answer is:
one-way
'unidirectional' is the definition.
(similar in meaning)
This is the entire clue.
(Other definitions for one-way that I've seen before include "Traffic sign" , "Moving in a single direction only" , "This street allows only single traffic flow" , "Moving in a sole direction" , "Allowing no return movement" .)