Fibre in light illuminated (5)
I believe the answer is:
rayon
'fibre' is the definition.
(I have seen 'Artificial fibre' mean 'rayon' so perhaps 'fibre' could also mean 'rayon')
'light illuminated' is the wordplay.
'light' becomes 'ray' (ray is a kind of light).
'illuminated' becomes 'on' (I can't explain this - if you can you should give a lot more credence to this answer).
'ray'+'on'='RAYON'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for rayon that I've seen before include "Imitation silk" , "Synthetic silk-like fabric" , "Man-made fibre" , "Or any (anag) -- fibre" , "Cellulose fabric" .)