Mark of the devil? (4)
I believe the answer is:
nick
This clue is a double definition.
'mark' is the first definition.
(I've seen this before)
'the devil?' is the second definition.
(Nick and Old Nick are names for the Devil)
'of' is the link.
(Other definitions for nick that I've seen before include "Catch" , "Notch; prison" , "Colloquially, a prison or police station" , "Little fellow" , "Lift" .)