The most common and most likely answer to this crossword clue is the 10 letter word NASTURTIUM
I've seen this crossword clue in The Daily Telegraph
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Buddy explains!

The straight part
We know that "Flower" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flower = nasturtium.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 NASTURTIUM mean?
NASTURTIUM noun- The popular name of any of the plants in the Tropaeolum genus of flowering plants native to south and central America.
- A plant in this genus, Tropaeolum majus.
- Any of the plants in the genus, Nasturtium, that includes watercress.