The most common and most likely answer to this crossword clue is the 3 letter word VAS
I've seen this crossword clue in Financial Times
V
A
S
Buddy explains!
The straight part
We know that "Tube" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tube = vas.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 VAS mean?
VAS noun- A vessel or duct transporting any bodily fluid, such as blood, lymph, chyle, or semen.
- (specifically) The vas deferens.