The most common and most likely answer to this crossword clue is the 6 letter word DECANT
I've seen this crossword clue in Financial Times
D
E
C
A
N
T
Buddy explains!

The straight part
We know that "Siphon off" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case siphon off = decant.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 DECANT mean?
DECANT verb- To pour off (a liquid) gently, so as not to disturb the sediment.
- To pour from one vessel into another.
- To flow.
- To remove a clone from its chamber, vat, or artificial womb.
- To rehouse people while their buildings are being refurbished or rebuilt.