The most common and most likely answer to this crossword clue is the 12 letter word AFTERTHOUGHT
I've seen this crossword clue in Financial Times
A
F
T
E
R
T
H
O
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G
H
T
More clues leading to the result AFTERTHOUGHT
- A later idea Mad Hatter fought
- Unhappily fatter, nevertheless tucking in? Here's a further idea
- My siblings are much older, and fatter, spoilt, if kept in
- A later train?
- Certainly not an original idea
- A reflection from worried father to thug
- Fatter, possibly eating whilst getting second opinion
- Fatter, out of shape, nevertheless tucking in? Here’s further idea
- Later addition making contribution to rafter, though temporary
Buddy explains!
The straight part
We know that "Certainly not an original idea" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case certainly not an original idea = afterthought.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 AFTERTHOUGHT mean?
AFTERTHOUGHT noun- A reflection after an act; a later or subsequent thought, action, or expedient.
- Something additional to the original plan or concept.
- (very rare except in the past tense, possibly nonstandard) To expound as an afterthought.
- To think about after the fact, reflect on.
- To repent, or feel remorse for; to regret.