Allowing travel in one direction only (3-3)
I believe the answer is:
one-way
'allowing travel in one direction only' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for one-way that I've seen before include "Unidirectional" , "Traffic sign" , "Single-directional like some streets" , "Like non-return valve" , "This street allows only single traffic flow" .)