Nasty fuels spilt outside mine (8)
I believe the answer is:
spiteful
'nasty' is the definition.
('spiteful' can be similar in meaning to 'nasty')
'fuels spilt outside mine' is the wordplay.
'spilt' is an anagram indicator.
'outside' is an insertion indicator.
'mine' becomes 'pit' (down the pit).
'fuels' anagrammed gives 'seful'.
'seful' placed around 'pit' is 'SPITEFUL'.
(Other definitions for spiteful that I've seen before include "Malevolent, wanting to hurt" , "Catty" , "Vengeful, malicious" , "Showing malice" , "Vindictive - malicious" .)