Fibre's length unchanged on rotation (5)
I believe the answer is:
sisal
'fibre's' is the definition.
(sisal is a kind of fibre)
'length unchanged on rotation' is the wordplay.
'length' becomes 'l' (abbreviation in geometry).
'unchanged' becomes 'as is' (as things currently are).
'on rotation' says the letters should be written in reverse.
'l'+'asis'='lasis'
'lasis' back-to-front is 'SISAL'.
(Other definitions for sisal that I've seen before include "Plant fibre used for ropes" , "constituent of rope" , "Rope fibre from leaves of tropical plant" , "Plant yielding rope fibre" , "Strong rope fibre" .)