The most common and most likely answer to this crossword clue is the 8 letter word CANNULAE
I've seen this crossword clue in The Guardian
C
A
N
N
U
L
A
E
Buddy explains!

The straight part
We know that "drains" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case drains = cannulae.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 CANNULAE mean?
CANNULAE noun- A tube inserted in the body to drain or inject fluid.
- A hose or tube that connects directly to an oxygen (O2) bottle/source from the user's nose, commonly used by aircraft pilots or others needing direct oxygen breathing apparatus.