A piece of grass 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)
'a piece of grass is a long' is the wordplay.
'a piece of' indicates a hidden word.
'SISAL' can be found hidden inside 'grass is a long'.
(Other definitions for sisal that I've seen before include "constituent of rope" , "material for painter, say?" , "Rope fibre from leaves of tropical plant" , "Agave yielding fibre" , "Plant fibre used to make string" .)