Odious fuel spilt on headgear (7)
I believe the answer is:
hateful
'odious' is the definition.
(similar in meaning)
'fuel spilt on headgear' is the wordplay.
'spilt' indicates anagramming the letters.
'on' indicates putting letters inside (as in clothing 'on' a person).
'headgear' becomes 'hat' (hats are a type of headgear).
'fuel' with letters rearranged gives 'eful'.
'eful' enclosing 'hat' is 'HATEFUL'.
(Other definitions for hateful that I've seen before include "Unpleasant" , "Loathsome" , "Dreadful" , "Detestable" , "Abhorrent" .)