The most common and most likely answer to this crossword clue is the 10 plus 5 letter word CALEDONIAN CANAL
I've seen this crossword clue in The Daily Telegraph
C
A
L
E
D
O
N
I
A
N
C
A
N
A
L
Buddy explains!
The straight part
We know that "waterway" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case waterway = caledonian canal.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.