Mark's causing alarm no end (4)
I believe the answer is:
scar
'mark's' is the definition.
('scar' can be a synonym of 'mark')
'alarm no end' is the wordplay.
'alarm' becomes 'scare' (I've seen this before).
'no end' means to remove the last letter.
'scare' with its final letter taken away is 'SCAR'.
'causing' is the link.
(Other definitions for scar that I've seen before include "Healed-cut mark" , "area of wound" , "It shows where wound was" , "Skin blemish - bare crag" , "evidence of damage" .)