The most common and most likely answer to this crossword clue is the 10 letter word SLAP-HAPPY
I've seen this crossword clue in The Guardian
S
L
A
P
-
H
A
P
P
Y
More clues leading to the result SLAP-HAPPY
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.