Diabolical elfin ran off (8)
I believe the answer is:
infernal
'diabolical' is the definition.
(I know that diabolical can be written as infernal)
'elfin ran off' is the wordplay.
'off' is an anagram indicator.
'elfin'+'ran'='elfinran'
'elfinran' anagrammed gives 'INFERNAL'.
(Other definitions for infernal that I've seen before include "awful" , "Of the nether regions" , "Feral inn - it's hellish" , "Blessed; damned" , "To do with hell and its fires" .)