Spitefully injured by litter (8)
I believe the answer is:
bitterly
'spitefully' is the definition.
Both the answer and definition are adverbs. Maybe there's an association between them I don't understand?
'injured by litter' is the wordplay.
'injured' indicates anagramming the letters.
'by'+'litter'='bylitter'
'bylitter' is an anagram of 'BITTERLY'.
Can you help me to learn more?
(Other definitions for bitterly that I've seen before include "Resentfully, rancorously" , "With resentment" , "Acrimoniously" , "In a resentful manner" , "Sharply, pungently" .)