Disappearance of brightness not good (4)
I believe the answer is:
loss
'disappearance' is the definition.
(I've seen this before)
'brightness not good' is the wordplay.
'brightness' becomes 'gloss' (gloss is a kind of brightness).
'not' is a deletion indicator.
'good' becomes 'g' (abbreviation).
'gloss' with 'g' taken away is 'LOSS'.
'of' acts as a link.
(Other definitions for loss that I've seen before include "Financial reverse" , "Deprivation; failure" , "Defeat; bereavement" , "Defeat (in sport)" , "Forfeiture" .)