The most common and most likely answer to this crossword clue is the 3 plus 6 letter word THE SOLENT
I've seen this crossword clue in Financial Times
T
H
E
S
O
L
E
N
T
More clues leading to the result THE SOLENT
Buddy explains!

The straight part
We know that "stretch of tidal water/r/e" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stretch of tidal water/r/e = the solent.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.