The most common and most likely answer to this crossword clue is the 7 letter word KIDDING
I've seen this crossword clue in The Times
K
I
D
D
I
N
G
More clues leading to the result KIDDING
Buddy explains!

The straight part
We know that "not being serious" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not being serious = kidding.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 KIDDING mean?
KIDDING verb- To make a fool of (someone).
- To dupe or deceive (someone).
- To make a joke with (someone).
- Of a goat, to give birth to kids.
- To joke.
- The action of the verb kid.
- The act of a goat giving birth.