The most common and most likely answer to this crossword clue is the 13 letter word COMPREHENSIVE
I've seen this crossword clue in The Daily Telegraph
C
O
M
P
R
E
H
E
N
S
I
V
E
More clues leading to the result COMPREHENSIVE
Buddy explains!

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 COMPREHENSIVE mean?
COMPREHENSIVE noun- A comprehensive school.
- Broadly or completely covering; including a large proportion of something.