The most common and most likely answer to this crossword clue is the 4 plus 4 letter word OPEN BOOK
I've seen this crossword clue in The Times
O
P
E
N
B
O
O
K
More clues leading to the result OPEN BOOK
Buddy explains!
The straight part
We know that "it’s readily understood" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s readily understood = open book.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.