The most common and most likely answer to this crossword clue is the 6 letter word TORULA
T
O
R
U
L
A
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The straight part
We know that "yeast" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case yeast = torula.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 TORULA mean?
TORULA noun- Any of a group of fungi, Candida utilis, related to the yeasts, sometimes used in processed food.
- A small torus.