The most common and most likely answer to this crossword clue is the 6 letter word THINGS
I've seen this crossword clue in The Daily Telegraph
T
H
I
N
G
S
Buddy explains!

The straight part
We know that "this stuff" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case this stuff = things.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 THINGS mean?
THINGS noun- That which is considered to exist as a separate entity, object, quality or concept.
- A word, symbol, sign, or other referent that can be used to refer to any entity.
- An individual object or distinct entity.
- A genuine concept, entity or phenomenon; something that actually exists (often contrary to expectation or belief).
- Whatever can be owned.
- Corporeal object.
- (somewhat obsolete, with the) The latest fad or fashion.
- (in the plural) Clothes, possessions or equipment.
- A unit or container, usually containing edible goods.
- A problem, dilemma, or complicating factor.
- A penis.
- A living being or creature.
- That which matters; the crux.
- Used after a noun to refer dismissively to the situation surrounding the noun's referent.
- That which is favoured; personal preference. (Used in possessive constructions.)
- A public assembly or judicial council in a Germanic country.
- A romantic relationship.
- To express as a thing; to reify.
- One's clothes, furniture, luggage, or possessions collectively; stuff