Main source of a flare-up (3)
I believe the answer is:
gas
'main' is the definition.
(gas main is a kind of main)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for gas that I've seen before include "Chat (colloq.)" , "Fuel; idle talk" , "Empty talk (colloq.)" , "It may be natural" , "Petrol (USA)" .)