The most common and most likely answer to this crossword clue is the 8 letter word FLIPPANT
I've seen this crossword clue in The Times
F
L
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P
P
A
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T
More clues leading to the result FLIPPANT
- To set program up in stone is not serious enough
- Disrespectful cheek — fine to gasp about that
- Breathe heavily after somersault that’s frivolous
- Get overexcited with wheeze that’s saucy
- Lose one’s rag before long, being facetious
- Cheek, being seen on pot in paper? It’s disrespectful
- Frivolous fellow with insolence getting to criticise team’s leader
- Drink, long and light
- After hot alcoholic drink take heavy breaths or shallow
- Irresponsible jerk with puff
Buddy explains!

The straight part
We know that "not serious enough" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not serious enough = flippant.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 FLIPPANT mean?
FLIPPANT adjective- Glib; speaking with ease and rapidity
- Nimble; limber.
- Showing disrespect through a casual attitude, levity, and a lack of due seriousness; pert.