Notice insubstantial illumination (9)
I believe the answer is:
spotlight
'illumination' is the definition.
(I have seen 'Focused illumination' mean 'spotlight' so perhaps 'illumination' could also mean 'spotlight')
'notice insubstantial' is the wordplay.
'notice' becomes 'spot' (spot can mean to notice or observe).
'insubstantial' becomes 'light' (I've seen this before).
'spot'+'light'='SPOTLIGHT'
(Other definitions for spotlight that I've seen before include "Where attention is focused" , "Prominent position" , "Illumination focused on small area" , "fame" , "Illuminate" .)