The most common and most likely answer to this crossword clue is the 5 plus 3 plus 3 plus 4 letter word REACH FOR THE MOON
I've seen this crossword clue in The Daily Telegraph
R
E
A
C
H
F
O
R
T
H
E
M
O
O
N
Buddy explains!
The straight part
We know that "Try something that looks impossible" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case try something that looks impossible = reach for the moon.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.