The most common and most likely answer to this crossword clue is the 11 letter word DICOTYLEDON
I've seen this crossword clue in The Times
D
I
C
O
T
Y
L
E
D
O
N
Buddy explains!

The straight part
We know that "flowering plant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flowering plant = dicotyledon.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 DICOTYLEDON mean?
DICOTYLEDON noun- A plant whose seedling has two cotyledons.
- Any plant in what used to be the Dicotyledones.