Initially get a faint source of illumination (8)
I believe the answer is:
gaslight
'source of illumination' is the definition.
(I've seen this before)
'initially get a faint' is the wordplay.
'initially' says to take the initial letters.
'faint' becomes 'slight' (slight can mean faint or small).
The first letter of 'get' is 'g'.
'g'+'a'+'slight'='GASLIGHT'
(Other definitions for gaslight that I've seen before include "that was illuminating for Fanny" , "Bergman film" , "Old film" , "Old guide in dark" , "Old form of illumination" .)