The most common and most likely answer to this crossword clue is the 7 plus 2 plus 4 letter word PROPHET OF DOOM
I've seen this crossword clue in The Guardian
P
R
O
P
H
E
T
O
F
D
O
O
M
Buddy explains!
The straight part
We know that "pessimist" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pessimist = prophet of doom.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.