The most common and most likely answer to this crossword clue is the 6 letter word HOSING
I've seen this crossword clue in The Times
H
O
S
I
N
G
More clues leading to the result HOSING
Buddy explains!

The straight part
We know that "Washing down" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case washing down = hosing.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 HOSING mean?
HOSING verb- To water or spray with a hose.
- To deliver using a hose.
- To provide with hose (garment)
- To attack and kill somebody, usually using a firearm.
- To trick or deceive.
- To break a computer so everything needs to be reinstalled; to wipe all files.
- To cause an unfair disadvantage to a player or team through poor officiating; especially, to cause a player or team to lose the game with an incorrect call.
- A spraying or washing with a hose.
- Material used for making hosiery.