The most common and most likely answer to this crossword clue is the 12 letter word NARCISSISTIC
I've seen this crossword clue in Financial Times
N
A
R
C
I
S
S
I
S
T
I
C
More clues leading to the result NARCISSISTIC
Buddy explains!
The straight part
We know that "showing complete disregard for others" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case showing complete disregard for others = narcissistic.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.
Crossword Answer definitions
NARCISSISTIC noun- A narcissist.
- Having an inflated idea of one's own importance.
- Obsessed with one's own self image and ego.