The most common and most likely answer to this crossword clue is the 12 letter word DEMONSTRABLE
I've seen this crossword clue in The Daily Telegraph
D
E
M
O
N
S
T
R
A
B
L
E
More clues leading to the result DEMONSTRABLE
Buddy explains!

The straight part
We know that "clear" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case clear = demonstrable.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 DEMONSTRABLE mean?
DEMONSTRABLE noun- Something that can be demonstrated.
- Able to be demonstrated.