Fibre used in optics is altered (5)
I believe the answer is:
sisal
'fibre' is the definition.
(I know that sisal is a type of plant fiber)
'in optics is altered' is the wordplay.
'in' indicates a hidden word.
'SISAL' is hidden within 'optics is altered'.
'used' is the link.
(Other definitions for sisal that I've seen before include "Sails (anag) - fibre" , "Plant yielding rope fibre" , "Agave fibre found in bad sails" , "material for painter, say?" , "constituent of rope" .)