The most common and most likely answer to this crossword clue is the 12 letter word LIGHT-HEADED
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More clues leading to the result LIGHT-HEADED
Buddy explains!

The straight part
We know that "without serious thought" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case without serious thought = light-headed.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 LIGHT-HEADED mean?
LIGHT-HEADED adjective- Disordered in the head; dizzy or feeling faint; hence, flighty in mind; delirious.
- Thoughtless; volatile; frivolous.