Good: a delicate form of illumination (8)
I believe the answer is:
gaslight
'form of illumination' is the definition.
(type of lighting)
'good a delicate' is the wordplay.
'good' becomes 'g' (abbreviation).
'delicate' becomes 'slight' (synonyms).
'g'+'a'+'slight'='GASLIGHT'
(Other definitions for gaslight that I've seen before include "Sort of illumination" , "Old guide in dark" , "Old film" , "Old form of illumination" , "Old-fashioned illumination" .)