The most common and most likely answer to this crossword clue is the 13 letter word SPECTROMETERS
I've seen this crossword clue in The Times
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Buddy explains!

The straight part
We know that "Instruments," is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case instruments, = spectrometers.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 SPECTROMETERS mean?
SPECTROMETERS noun- An optical instrument for measuring the absorption of light by chemical substances; typically it will plot a graph of absorption versus wavelength or frequency, and the patterns produced are used to identify the substances present, and their internal structure.