The most common and most likely answer to this crossword clue is the 5 letter word OAKUM
I've seen this crossword clue in The Times
O
A
K
U
M
More clues leading to the result OAKUM
- Ropy stuff for sealing wood, I’m thinking
- Ropy material from Rutland town, so to speak
- East Ender’s claptrap about a loose fibre
- Chinese leader goes round country backing tarred rope
- What’ll help one remain seaworthy? Sounds like ’ogwash
- Strand of rope obtained from tree on the foothills of Kathmandu and Sikkim
- All right to have a lout discarding black caulking material
- Timber, erm, material once common in joints for seafarers
Buddy explains!

The straight part
We know that "loose fibre" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case loose fibre = oakum.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 OAKUM mean?
OAKUM noun- A material, consisting of tarred fibres, used to caulk or pack joints in plumbing, masonry, and wooden shipbuilding.
- The coarse portion separated from flax or hemp in hackling.