A blow for the devil to strike! (6)
I believe the answer is:
impact
'to strike' is the definition.
(I've seen this before)
'a blow for the devil' is the wordplay.
'a blow' becomes 'act' (I am not sure about this - if you are sure you should believe this answer much more).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'the devil' becomes 'imp' (devilish creature).
'act' after 'imp' is 'IMPACT'.
Can you help me to learn more?
(Other definitions for impact that I've seen before include "Bear upon" , "Collision - effect" , "Serious effect" , "Force of blow or collision" , "Force exerted by collision" .)