The most common and most likely answer to this crossword clue is the 5 plus 5 letter word GLIAL CELLS
I've seen this crossword clue in The Guardian
G
L
I
A
L
C
E
L
L
S
Buddy explains!
The straight part
We know that "brain matter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case brain matter = glial cells.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.