The most common and most likely answer to this crossword clue is the 15 letter word TONGUE-IN-CHEEK
I've seen this crossword clue in The Guardian
T
O
N
G
U
E
-
I
N
-
C
H
E
E
K
More clues leading to the result TONGUE-IN-CHEEK
- Playful language getting not very far — yikes!
- Language fashionable — impudence intended to be humorous
- Chaincap, sausance or lipalip, say? Only joking!
- Perhaps English eke out small amount to be eaten: that’s ironic
- Ironic language, at church, before my god
- Get union involved with 10? It’s not serious
- Perhaps English eke out small amount to be eaten: that's ironic
- Facetious language followed by creep with cry of mock horror
Buddy explains!
The straight part
We know that "not serious" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not serious = tongue-in-cheek.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 TONGUE-IN-CHEEK mean?
TONGUE-IN-CHEEK adjective- Not intended seriously; jocular or humorous.
- With contempt.
- With irony.