The most common and most likely answer to this crossword clue is the 8 letter word BURETTES
I've seen this crossword clue in The Guardian
B
U
R
E
T
T
E
S
Buddy explains!

The straight part
We know that "Glass tubes/Glass tubes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case glass tubes/glass tubes = burettes.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 BURETTES mean?
BURETTES noun- A glass tube with fine gradations and a stopcock at the bottom, used in laboratory procedures for accurate fluid dispensing and titration.
- An altar cruet.