Fibre found in grass is allotropic (5)
I believe the answer is:
sisal
'fibre' is the definition.
(sisal is a kind of fibre)
'in grass is allotropic' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SISAL' is hidden in the letters of 'grass is allotropic'.
'found' acts as a link.
(Other definitions for sisal that I've seen before include "Type of fibre" , "Plant yielding rope fibre" , "Rope-fibre made from leaves of tropical plant" , "Fibre used in rope-making" , "Agave rope-fibre" .)