Knowing about luminescence? Not very much (8)
I believe the answer is:
slightly
'not very much' is the definition.
(I've seen this before)
'knowing about luminescence?' is the wordplay.
'knowing' becomes 'sly' (I've seen this in another clue).
'about' indicates putting letters inside.
'luminescence?' becomes 'light' (luminescence is a kind of light).
'sly' going around 'light' is 'SLIGHTLY'.
(Other definitions for slightly that I've seen before include "Adverb meaning slender or slim" , "Not very much" , "To a small extent" , "Mildly" , "Quite!" .)