The most common and most likely answer to this crossword clue is the 6 letter word FATTEN
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More clues leading to the result FATTEN
Buddy explains!

The straight part
We know that "stuff" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stuff = fatten.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 FATTEN mean?
FATTEN verb- To cause (a person or animal) to be fat or fatter.
- (of a person or animal) To become fat or fatter.
- To make thick or thicker (something containing paper, often money).
- To become thick or thicker.
- To make (soil) fertile and fruitful.
- To become fertile and fruitful.