The most common and most likely answer to this crossword clue is the 2 plus 10 letter word ON REFLECTION
I've seen this crossword clue in The Guardian
O
N
R
E
F
L
E
C
T
I
O
N
Buddy explains!
The straight part
We know that "when one’s thought about it" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case when one’s thought about it = on reflection.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.