Bad local I repeatedly slammed as hellish (10)
I believe the answer is:
diabolical
'hellish' is the definition.
(synonyms)
'bad local i repeatedly slammed' is the wordplay.
'i repeatedly' can mean two substitutions for 'i'.
'slammed' indicates anagramming the letters.
'bad'+'local'+'i'+'i'='badlocalii'
'badlocalii' anagrammed gives 'DIABOLICAL'.
'as' acts as a link.
(Other definitions for diabolical that I've seen before include "iniquitous" , "evil" , "Devilish" , "Absolute" , "Demonic" .)