The most common and most likely answer to this crossword clue is the 13 letter word COMPREHENSION
I've seen this crossword clue in The Guardian
C
O
M
P
R
E
H
E
N
S
I
O
N
Buddy explains!
The straight part
We know that "understanding" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case understanding = comprehension.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 COMPREHENSION mean?
COMPREHENSION noun- Thorough understanding
- The totality of intensions, that is, attributes, characters, marks, properties, or qualities, that the object possesses, or else the totality of intensions that are pertinent to the context of a given discussion.
- A compact syntax for generating a list in some functional programming languages.
- The inclusion of nonconformists within the Church of England.