Cryptic crossword clue
Discoverer of polarised light almost complimentary backing optical instrument 7

The most common and most likely answer to this crossword clue is the 7 letter word FRESNEL

I've seen this crossword clue in Financial Times
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Buddy explains!

The straight part
We know that "Discoverer of polarised light" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case discoverer of polarised light = fresnel.
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 FRESNEL mean?

FRESNEL noun
  1. A unit of frequency equal to 1012 hertz, or one terahertz.
  2. A Fresnel lens or a light feature using such a lens

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