Regularly warning of unknown bug (5)
I believe the answer is:
annoy
'bug' is the definition.
(I know that bug is a more specific form of bedevil)
'regularly warning of unknown' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'unknown' becomes 'y' (algebra).
'warning'+'of'='warningof'
The alternate letters of 'warningof' are 'anno'.
'anno'+'y'='ANNOY'
(Other definitions for annoy that I've seen before include "trouble" , "Nark - nettle" , "Irritate - pester" , "Vex" , "Gall" .)