The most common and most likely answer to this crossword clue is the 7 letter word FERMENT
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Buddy explains!

The straight part
We know that "Brew" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case brew = ferment.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 FERMENT mean?
FERMENT noun- Something, such as a yeast or barm, that causes fermentation.
- A state of agitation or of turbulent change.
- A gentle internal motion of the constituent parts of a fluid; fermentation.
- A catalyst.
- To react, using fermentation; especially to produce alcohol by aging or by allowing yeast to act on sugars; to brew.
- To stir up, agitate, cause unrest or excitement in.