Light brown coal? A non-starter (6)
I believe the answer is:
ignite
'light' is the definition.
(I've seen this before)
'brown coal? a non-starter' is the wordplay.
'brown coal?' becomes 'lignite' (synonyms).
'a non-starter' means to remove the first letter (the letter which starts the word is ignored).
'lignite' with its initial letter taken away is 'IGNITE'.
(Other definitions for ignite that I've seen before include "Combust" , "Swan constellation" , "Detonate" , "Cause to start burning" , "Cause to catch fire" .)