The most common and most likely answer to this crossword clue is the 8 letter word DREAMING
I've seen this crossword clue in The Sunday Telegraph
D
R
E
A
M
I
N
G
Buddy explains!

The straight part
We know that "Absent-minded" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case absent-minded = dreaming.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 DREAMING mean?
DREAMING verb- To see imaginary events in one's mind while sleeping.
- To hope, to wish.
- To daydream.
- To envision as an imaginary experience (usually when asleep).
- To consider the possibility (of).
- An instance of dreaming; a dream or reverie.