Dress up as Satan, passing masks on (8)
I believe the answer is:
demonise
'dress up as satan' is the definition.
I can't tell whether this definition defines the answer.
'passing masks on' is the wordplay.
'passing' becomes 'demise' (I've seen this before).
'masks' indicates putting letters inside (mask can mean to cover up or conceal).
'demise' placed around 'on' is 'DEMONISE'.
Can you help me to learn more?
(Other definitions for demonise that I've seen before include "Portray as evil" , "Blacken name" , "Make out to be evil" .)