The most common and most likely answer to this crossword clue is the 10 letter word MICROMETER
I've seen this crossword clue in The Times
M
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The straight part
We know that "gauge" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gauge = micrometer.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 MICROMETER mean?
MICROMETER noun- An SI/MKS unit of measure, the length of one millionth of a meter. Symbols: µm, um, rm.
- A device used to measure distance very precisely but within a limited range, especially depth, thickness, and diameter.