Nothing different -- always moving in a single direction (3-3)
I believe the answer is:
one-way
'moving in a single direction' is the definition.
(I've seen this before)
'nothing different always' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'different' becomes 'new' ('new' can be similar in meaning to 'different').
'always' becomes 'ay'.
'o'+'new'+'ay'='ONE-WAY'
(Other definitions for one-way that I've seen before include "This road permits only a single flow of traffic" , "Kind of street where cars shouldn't meet head-on" , "Single-directional like some streets" , "Not allowing return motion" , "Describing a street where vehicles shouldn't meet head-on" .)