The most common and most likely answer to this crossword clue is the 10 letter word FATHOMETER
F
A
T
H
O
M
E
T
E
R
Buddy explains!
The straight part
We know that "instrument" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case instrument = fathometer.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.
What does FATHOMETER mean?
FATHOMETER noun- A depth finder that uses sound waves to determine the depth of water.