Fuel for spontaneous chat (7,3)
I believe the answer is:
natural gas
'fuel' is the definition.
(natural gas is a kind of fuel)
'spontaneous chat' is the wordplay.
'spontaneous' becomes 'natural' ('natural' can be a synonym of 'spontaneous').
'chat' becomes 'gas' (informal term meaning to chat).
'natural'+'gas'='NATURAL GAS'
'for' is the link.
(Other definitions for natural gas that I've seen before include "Fuel from eg 2 down" , "Fuel from underground" , "Energy source from underground" .)