Light source good with volatile plasma (3,4)
I believe the answer is:
gas lamp
'light source' is the definition.
(thesaurus)
'good with volatile plasma' is the wordplay.
'good' becomes 'g' (abbreviation).
'with' is a charade indicator (letters next to each other).
'volatile' indicates anagramming the letters.
'plasma' anagrammed gives 'aslamp'.
'g'+'aslamp'='GAS-LAMP'
(Other definitions for gas lamp that I've seen before include "Old illuminator" , "Source of illumination" , "Burning source of light" , "lighter" , "Bright object" .)