The most common and most likely answer to this crossword clue is the 6 letter word BUNGEE
I've seen this crossword clue in The Guardian
B
U
N
G
E
E
More clues leading to the result BUNGEE
Buddy explains!

The straight part
We know that "a flexible rope" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a flexible rope = bungee.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 BUNGEE mean?
BUNGEE noun- An elastic fabric-bound strap with a hook at each end, used for securing luggage.
- An elastic cord tied to the ankles of the jumper in bungee jumping.
- A rubber eraser.