The most common and most likely answer to this crossword clue is the 7 letter word FOXTAIL
I've seen this crossword clue in Financial Times
F
O
X
T
A
I
L
Buddy explains!
The straight part
We know that "Brush" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case brush = foxtail.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 FOXTAIL mean?
FOXTAIL noun- The tail of a fox.
- A dry spikelet or spikelet seed and flower cluster of some grasses
- A plant having a part resembling the tail of a fox or such spikelet.
- The last cinders obtained in the fining process.