One possessed by devil is after a large dog (8)
I believe the answer is:
alsatian
'dog' is the definition.
(I know that Alsatian is a type of dog)
'one possessed by devil is after a large' is the wordplay.
'one' becomes 'i' (Roman numeral).
'possessed by' is an insertion indicator.
'devil' becomes 'satan' (synonyms).
'is after' is a charade indicator (letters next to each other).
'large' becomes 'L' (eg in clothes sizes).
'i' going into 'satan' is 'satian'.
'a'+'l'='al'
'satian' put after 'al' is 'ALSATIAN'.
(Other definitions for alsatian that I've seen before include "Wolf-like dog" , "Old Frenchman" , "Large dog and person from northern France" , "Large canine, may be police dog" , "Strong dog known as German Shepherd" .)