The most common and most likely answer to this crossword clue is the 9 letter word DOSIMETER
I've seen this crossword clue in The Times
D
O
S
I
M
E
T
E
R
Buddy explains!

The straight part
We know that "Scientific instrument" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case scientific instrument = dosimeter.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 DOSIMETER mean?
DOSIMETER noun- A device used to measure a dose of ionizing radiation.