Light on artificial silk (5)
I believe the answer is:
rayon
'artificial silk' is the definition.
(I've seen this before)
'light on' is the wordplay.
'light' becomes 'ray' (ray is a kind of light).
'ray'+'on'='RAYON'
(Other definitions for rayon that I've seen before include "Man-made fabric" , "Silklike material" , "Synthetic fibre" , "Cellulose textile fibre" , "Artificial material" .)