The most common and most likely answer to this crossword clue is the 10 plus 6 letter word POISON-PEN LETTER
I've seen this crossword clue in Independent
P
O
I
S
O
N
-
P
E
N
L
E
T
T
E
R
More clues leading to the result POISON-PEN LETTER
Buddy explains!
The straight part
We know that "&lit? – open (anonymous) note" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case &lit? – open (anonymous) note = poison-pen letter.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.