Fusion era may turn out to be horrible (9)
I believe the answer is:
nefarious
'horrible' is the definition.
(thesaurus)
'fusion era may turn out to be' is the wordplay.
'may turn out to be' indicates an anagram.
'fusion'+'era'='fusionera'
'fusionera' anagrammed gives 'NEFARIOUS'.
(Other definitions for nefarious that I've seen before include "Extremely wicked" , "Villainous" , "immoral?" , "Very bad" , "Wicked, criminal" .)