The most common and most likely answer to this crossword clue is the 4 letter word WOAD
W
O
A
D
Buddy explains!

The straight part
We know that "dyestuff" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dyestuff = woad.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 WOAD mean?
WOAD noun- The plant Isatis tinctoria.
- The blue dye made from the leaves of the plant.
- To plant or cultivate woad.
- To dye with woad.