The most common and most likely answer to this crossword clue is the 8 letter word ADAPTIVE
I've seen this crossword clue in The Times
A
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More clues leading to the result ADAPTIVE
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The straight part
We know that "Flexible" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flexible = adaptive.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 ADAPTIVE mean?
ADAPTIVE adjective- Of, pertaining to, characterized by or showing adaptation; making or made fit or suitable.
- Capable of being adapted or of adapting; susceptible of or undergoing accordant change.
- Of a trait: that helps an individual to function well in society.