Cryptic crossword clue
I enter for merest tweaks in measuring instruments 15

The most common and most likely answer to this crossword clue is the 15 letter word INTERFEROMETERS

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Buddy explains!

The straight part
We know that "measuring instruments" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case measuring instruments = interferometers.
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.

Crossword Answer definitions

INTERFEROMETERS noun
  1. Any of several instruments that use the interference of waves to determine wavelengths and wave velocities, determine refractive indices, measure small distances, temperature changes, stresses, and many other useful measurements.

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