The most common and most likely answer to this crossword clue is the 8 letter word MENISCUS
I've seen this crossword clue in The Times
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Buddy explains!
The straight part
We know that "Evidence of tension in the Tube?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case evidence of tension in the tube? = meniscus.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 MENISCUS mean?
MENISCUS noun- A crescent moon, or an object shaped like it.
- A lens which is convex on one side and concave on the other, being crescent-shaped in cross-section.
- The curved surface of liquids in tubes, whether concave or convex, caused by the surface tension of the liquid.
- Either of two parts of the human knee that provide structural integrity to the knee when it undergoes tension and torsion.