The most common and most likely answer to this crossword clue is the 8 letter word EYEPIECE
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Buddy explains!
The straight part
We know that "part of an optical instrument" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of an optical instrument = eyepiece.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 EYEPIECE mean?
EYEPIECE noun- The lens (or combination of lenses) at the eye end of a microscope or telescope by which the image is viewed.