Boil treated with acid? That’s fiendish (8)
I believe the answer is:
diabolic
'fiendish' is the definition.
(I know that fiendish can be written as diabolic)
'boil treated with acid?' is the wordplay.
'treated with' indicates anagramming the letters.
'boil' mixed up with 'acid' makes 'DIABOLIC'.
'that's' is the link.
(Other definitions for diabolic that I've seen before include "Of the devil" , "Extremely cruel" , "Fiendish and cruel, like Old Nick" , "Devilish" , "wicked" .)