Inside shorts is a lengthy fibre (5)
I believe the answer is:
sisal
'fibre' is the definition.
(I know that sisal is a type of plant fiber)
'inside shorts is a lengthy' is the wordplay.
'inside' says the answer is hidden in the clue.
'SISAL' can be found hidden inside 'shorts is a lengthy'.
(Other definitions for sisal that I've seen before include "Plant yielding rope fibre" , "constituent of rope" , "Fibre used in rope-making" , "Plant fibre used for making rope" , "Fibrous plant used for rope" .)