The most common and most likely answer to this crossword clue is the 6 plus 2 plus 3 plus 6 letter word ASLEEP AT THE SWITCH
I've seen this crossword clue in Financial Times
A
S
L
E
E
P
A
T
T
H
E
S
W
I
T
C
H
Buddy explains!
The straight part
We know that "not paying attention" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not paying attention = asleep at the switch.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.