Piece of floss is a long fibre (5)
I believe the answer is:
sisal
'fibre' is the definition.
(I know that sisal is a type of plant fiber)
'piece of floss is a long' is the wordplay.
'piece of' indicates the answer is hidden within the clue.
'SISAL' is hidden in the letters of 'floss is a long'.
(Other definitions for sisal that I've seen before include "Type of fibre" , "material for painter, say?" , "Fibre-yielding agave" , "Plant yielding rope fibre" , "Ropemaker" .)