Plentiful spillage of fuel can be very serious (7)
I believe the answer is:
fateful
'serious' is the definition.
(thesaurus)
'plentiful spillage of fuel can be very' is the wordplay.
I cannot really understand how this works, but
an anagram of 'fuel' is 'eful' which is within the answer.
The remaining letters 'fat' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for fateful that I've seen before include "critical" , "Prophetic - portentous" , "Portentous; bringing death" , "Dire in its implications" , "Decisive" .)