Falling in fuel by chance (12)
I believe the answer is:
coincidental
'by chance' is the definition.
(I've seen this before)
'falling in fuel' is the wordplay.
'falling' becomes 'incident' (I've seen this before).
'in' means one lot of letters goes inside another.
'fuel' becomes 'coal' (coal is a kind of fuel).
'incident' going into 'coal' is 'COINCIDENTAL'.
(Other definitions for coincidental that I've seen before include "Happening at the same time" , "Apparently randomly concurrent" , "Simultaneous" , "Happening by chance" , "fortuitous" .)