The most common and most likely answer to this crossword clue is the 5 letter word UMBEL
I've seen this crossword clue in The Times
U
M
B
E
L
Buddy explains!
The straight part
We know that "flower cluster" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flower cluster = umbel.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 UMBEL mean?
UMBEL noun- A flat-topped or rounded flower-cluster (= inflorescence) in which the individual flower stalks arise from the same point, the youngest flowers being at the centre.