The most common and most likely answer to this crossword clue is the 12 letter word ILL-INFORMED
I've seen this crossword clue in The Guardian
I
L
L
-
I
N
F
O
R
M
E
D
Buddy explains!
The straight part
We know that "Clueless" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case clueless = ill-informed.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 ILL-INFORMED mean?
ILL-INFORMED adjective- Poorly informed; ignorant.