The most common and most likely answer to this crossword clue is the 5 letter word SISAL
I've seen this crossword clue in Financial Times
S
I
S
A
L
More clues leading to the result SISAL
- Getting up, 5 consumes first of its fibre
- Rope-maker’s sibling briefly linked with notorious gangster
- Fibrous plant is invading South American lake
- A girl suffering setback needs the ultimate in moral fibre
- Fibre in diets is a laxative
- Maiden crossing island brought back hemp, of a sort
- Retiring girl eating last of broccoli for fibre
- Fibre one’s covered with tar mostly
- Rope-maker extremely sceptical about savings scheme
- Fibre optics is altering displays
Buddy explains!

The straight part
We know that "plant fibre" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plant fibre = sisal.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 SISAL mean?
SISAL noun- A Central American plant, Agave sisalana, cultivated for its sword-shaped leaves that yield fibers used for rope.
- The fibre of the plant.
- A sisal mat.