Demon wind damage (6)
I believe the answer is:
impair
'damage' is the definition.
(impairing is a kind of damaging)
'demon wind' is the wordplay.
'demon' becomes 'imp' (I've seen this before).
'wind' becomes 'air' (air is a kind of wind).
'imp'+'air'='IMPAIR'
(Other definitions for impair that I've seen before include "Hinder" , "blunt" , "Hobble" , "Spoil, degrade" , "Harm" .)