The most common and most likely answer to this crossword clue is the 3 plus 2 plus 2 plus 3 letter word AND SO TO BED
I've seen this crossword clue in The Daily Telegraph
A
N
D
S
O
T
O
B
E
D
Buddy explains!
The straight part
We know that "diarist’s last words" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case diarist’s last words = and so to bed.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.