The most common and most likely answer to this crossword clue is the 3 plus 5 plus 7 letter word THE PENNY DROPPED
I've seen this crossword clue in The Times
T
H
E
P
E
N
N
Y
D
R
O
P
P
E
D
More clues leading to the result THE PENNY DROPPED
Buddy explains!
The straight part
We know that "what happened around 10?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what happened around 10? = the penny dropped.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.