The most common and most likely answer to this crossword clue is the 10 letter word UNTHINKING
I've seen this crossword clue in The Guardian
U
N
T
H
I
N
K
I
N
G
More clues leading to the result UNTHINKING
Buddy explains!

The straight part
We know that "Thoughtless" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thoughtless = unthinking.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 UNTHINKING mean?
UNTHINKING verb- To undo the process of thinking.
- Without proper thought; thoughtless.
- Showing no regard; careless or unconcerned.