Nightmares including tumbril on a regular basis (6)
I believe the answer is:
incubi
'nightmares' is the definition.
('incubus' can be a synonym of 'nightmare')
'including tumbril on a regular basis' is the wordplay.
'including' becomes 'inc'.
'on a regular basis' means one should take alternating letters.
The alternating letters of 'tumbril' are 'ubi'.
'inc'+'ubi'='INCUBI'
(Other definitions for incubi that I've seen before include "nightmares" , "Demons of the night" .)