The most common and most likely answer to this crossword clue is the 3 plus 6 letter word AHA MOMENT
I've seen this crossword clue in The Guardian
A
H
A
M
O
M
E
N
T
More clues leading to the result AHA MOMENT
- Occasion for Penny to drop Mark sign, suitably attired for wedding?
- Man at home is excited getting it
- An actor warning cast finally, when he twigs
- Puzzled man at home when the penny drops
- The penny drops when this clue is solved by an amateur probably? Not me!
- What Archimedes had man at home put in order
Buddy explains!
The straight part
We know that "when the penny drops" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case when the penny drops = aha moment.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.