Talk on fair's source of illumination (8)
I believe the answer is:
gaslight
'source of illumination' is the definition.
(I've seen this before)
'talk on fair's' is the wordplay.
'talk' becomes 'gas' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'fair' becomes 'light' (I've seen this before**).
'gas'+'light'='GASLIGHT'
(Other definitions for gaslight that I've seen before include "Old guide in dark" , "Bergman film" , "source of illumination" , "Old form of illumination" , "that was illuminating for Fanny" .)