Sign of doom seen regularly (4)
I believe the answer is:
omen
'sign' is the definition.
(omen is a kind of sign)
'doom seen regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'doom'+'seen'='doomseen'
The alternate letters of 'doomseen' are 'OMEN'.
'of' is the link.
(Other definitions for omen that I've seen before include "Gloomy sign" , "Portent of things to come" , "writing on the wall" , "premonition" , "Forewarning, harbinger" .)