The most common and most likely answer to this crossword clue is the 3 plus 2 plus 8 letter word LAW OF AVERAGES
I've seen this crossword clue in Independent
L
A
W
O
F
A
V
E
R
A
G
E
S
Buddy explains!
The straight part
We know that "mean observation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mean observation = law of averages.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.