The most common and most likely answer to this crossword clue is the 7 letter word GENISTA
I've seen this crossword clue in Independent
G
E
N
I
S
T
A
Buddy explains!

The straight part
We know that "broom" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case broom = genista.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 GENISTA mean?
GENISTA noun- Any member of the flowering plant genus Genista.