The most common and most likely answer to this crossword clue is the 9 letter word SOLEMNISE
I've seen this crossword clue in Financial Times
S
O
L
E
M
N
I
S
E
Buddy explains!

The straight part
We know that "Observe" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case observe = solemnise.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 SOLEMNISE mean?
SOLEMNISE verb- To make solemn, or official, through ceremony or legal act.
- To make grave, serious, and reverential.