The most common and most likely answer to this crossword clue is the 5 plus 4 plus 4 letter word ABOVE ONES HEAD
I've seen this crossword clue in The Times
A
B
O
V
E
O
N
E
S
H
E
A
D
More clues leading to the result ABOVE ONES HEAD
Buddy explains!
The straight part
We know that "Cap may be/impossible to understand" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cap may be/impossible to understand = above ones head.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.