Settled down with article on Icelandic case - it's diabolical (7)
I believe the answer is:
satanic
'it's diabolical' is the definition.
(synonyms)
'settled down with article on icelandic case' is the wordplay.
'settled down' becomes 'sat' (I've seen this in another clue).
'with' is a charade indicator (letters next to each other).
'article' becomes 'an' (English indefinite article).
'on' is a charade indicator (letters next to each other).
'case' says to hollow out the word (remove centre letters).
'icelandic' with its middle removed is 'ic'.
'sat'+'an'+'ic'='SATANIC'
(Other definitions for satanic that I've seen before include "Nick's" , "Supremely evil" , "Relating to the devil" , "Devilish (as Blake's 18)" , "Devilish, like Blake's mills" .)