Terribly bad, bad local I repeatedly trashed (10)
I believe the answer is:
diabolical
'terribly bad' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I don't see?
'bad local i repeatedly trashed' is the wordplay.
'i repeatedly' can mean two substitutions for 'i'.
'trashed' is an anagram indicator.
'bad'+'local'+'i'+'i'='badlocalii'
'badlocalii' anagrammed gives 'DIABOLICAL'.
Can you help me to learn more?
(Other definitions for diabolical that I've seen before include "Absolute" , "Devilish" , "shocking" , "evil" , "Fiendish" .)