The most common and most likely answer to this crossword clue is the 5 letter word ISTLE
I've seen this crossword clue in The Times
I
S
T
L
E
More clues leading to the result ISTLE
- Fibre in lifebelts is rejected
- Man, say, embracing diet finally providing fibre
- Fibre for carpet tiles recycled
- Man e.g. given time inside showing fibre
- Most of fine – man weaves – in filaments perhaps
- Son in book decapitated? We may have to thank Agave for this!
- Fibrous stuff extractable from mistletoe
- First to join ends of optical nerve fibre
- Felt silly returning some fibre
- Fibre in bristles
Buddy explains!

The straight part
We know that "fibre" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fibre = istle.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 ISTLE mean?
ISTLE noun- A variety of Agave angustifolia var. angustifolia (syn. Agave rigida), furnishing a strong coarse fiber.
- The fiber itself; pita, or Tampico fiber.