The most common and most likely answer to this crossword clue is the 4 plus 6 letter word LOCH CRINAN
I've seen this crossword clue in The Guardian
L
O
C
H
C
R
I
N
A
N
Buddy explains!
The straight part
We know that "water in the Highlands/water in the Highlands" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case water in the highlands/water in the highlands = loch crinan.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.