The most common and most likely answer to this crossword clue is the 7 letter word TETRODE
I've seen this crossword clue in The Times
T
E
T
R
O
D
E
More clues leading to the result TETRODE
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 = tetrode.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 TETRODE mean?
TETRODE noun- A thermionic valve similar to a triode with the addition of a screen grid to protect the control grid.
- A dynatron.
- A dual-gate MOSFET.