The most common and most likely answer to this crossword clue is the 11 letter word SLAP-HAPPY]
I've seen this crossword clue in The Guardian
S
L
A
P
-
H
A
P
P
Y
]
Buddy explains!
The straight part
We know that "Careless" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case careless = slap-happy].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 SLAP-HAPPY] mean?
SLAP-HAPPY] adjective- Incoherent from being struck; punch drunk.
- In a humorous mood as a result of fatigue; silly from being tired.
- Irresponsibly free.