Mark is almost frightening (4)
I believe the answer is:
scar
'mark' is the definition.
('scar' can be a synonym of 'mark')
'almost frightening' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'frightening' becomes 'scary' (I've seen this before).
'scary' with its final letter taken off is 'SCAR'.
'is' is the link.
(Other definitions for scar that I've seen before include "Permanent mark left by a wound" , "Mark, blemish" , "A bare precipice" , "showing effect of damage to body" , "Sign of wound" .)