The most common and most likely answer to this crossword clue is the 6 plus 11 letter word SINGLE CARRIAGEWAY
I've seen this crossword clue in The Daily Telegraph
S
I
N
G
L
E
C
A
R
R
I
A
G
E
W
A
Y
Buddy explains!
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.