The most common and most likely answer to this crossword clue is the 9 letter word HELIOSTAT
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More clues leading to the result HELIOSTAT
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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 = heliostat.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 HELIOSTAT mean?
HELIOSTAT noun- A device that includes a plane mirror which turns so as to keep reflecting sunlight toward a predetermined target, compensating for the sun's apparent motions in the sky. The target may be a physical object, distant from the heliostat, or a direction in space, and is almost always stationary relative to the heliostat, so the light is reflected in a fixed direction.