The most common and most likely answer to this crossword clue is the 9 letter word COMPENDIA
I've seen this crossword clue in The Times
C
O
M
P
E
N
D
I
A
Buddy explains!

The straight part
We know that "Collected material" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case collected material = compendia.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 COMPENDIA mean?
COMPENDIA noun- A short, complete summary; an abstract.
- A list or collection of various items.