The most common and most likely answer to this crossword clue is the 4 letter word INKS
I
N
K
S
Buddy explains!

The straight part
We know that "Writing materials" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case writing materials = inks.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 INKS mean?
INKS noun- A pigment (or dye)-based fluid used for writing, printing etc.
- A particular type, color or container of this fluid.
- The black or dark-colored fluid ejected by squid, octopus etc, as a protective strategy.
- Publicity.
- Tattoo work.
- Cheap red wine.
- To apply ink to; to cover or smear with ink.
- To sign (a contract or similar document).
- To apply a tattoo to (someone).
- (of a squid or octopus) to eject ink (sense 3)