Devil in Lucifer as callous (6)
I believe the answer is:
rascal
'devil' is the definition.
(thesaurus)
'in lucifer as callous' is the wordplay.
'in' indicates a hidden word.
'RASCAL' is hidden within 'lucifer as callous'.
(Other definitions for rascal that I've seen before include "Brat" , "Scallywag" , "not one to be admired" , "naughty child" , "Knave, rogue" .)