The most common and most likely answer to this crossword clue is the 5 letter word YEAST
Y
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A
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More clues leading to the result YEAST
- Cause of ferment, Irish poet turning tail
- Fungus certainly turns up after vase is removed
- The old street accommodates a brewery agent
- When put into still it helps make alcohol
- Fungus still covering wings of aphids
- Brewing ingredient? It's put back in still
- For riser, inactivity's ending — whence comes the dawn?
- Raising agent from Axis, not the West
- Bread essential for another poet Eliot upset
- It gives rise to unusually easy time
Buddy explains!

The straight part
We know that "brewery agent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case brewery agent = yeast.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 YEAST mean?
YEAST noun- An often humid, yellowish froth produced by fermenting malt worts, and used to brew beer, leaven bread, and also used in certain medicines.
- A single-celled fungus of a wide variety of taxonomic families.
- A frothy foam.
- To ferment.
- (of something prepared with a yeasted dough) To rise.
- To exaggerate