The most common and most likely answer to this crossword clue is the 12 letter word BRAIN-TEASER
I've seen this crossword clue in Independent
B
R
A
I
N
-
T
E
A
S
E
R
Buddy explains!
The straight part
We know that "Poser" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case poser = brain-teaser.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 BRAIN-TEASER mean?
BRAIN-TEASER noun- A complex riddle or puzzle.