The most common and most likely answer to this crossword clue is the 7 plus 5 letter word TRAFFIC LIGHT
I've seen this crossword clue in The Times
T
R
A
F
F
I
C
L
I
G
H
T
Buddy explains!
The straight part
We know that "one's seen on the street" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one's seen on the street = traffic light.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.