The most common and most likely answer to this crossword clue is the 6 letter word LAVABO
I've seen this crossword clue in The Guardian
L
A
V
A
B
O
More clues leading to the result LAVABO
Buddy explains!

The straight part
We know that "Washbasin" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case washbasin = lavabo.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 LAVABO mean?
LAVABO noun- A ritual involving the washing of one's hands at a church's offertory before handling the Eucharist.
- The small towel used to dry the priest's hands following the ritual.
- A washbasin, particularly the one in a church used in the ritual.
- A trough used for washing at some medieval monasteries.
- A lavatory: a room used for urination and defecation.