Dull Mondays regularly boring princess (7)
I believe the answer is:
anodyne
'dull' is the definition.
(I've seen this before)
'mondays regularly boring princess' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'boring' means one lot of letters goes inside another (to bore something can mean to drill into it).
'princess' becomes 'Anne' (Princess Anne, daughter of Queen Elizabeth).
The alternating letters of 'mondays' are 'ody'.
'ody' put within 'anne' is 'ANODYNE'.
(Other definitions for anodyne that I've seen before include "Annoyed with pain-relieving drug" , "Innocent" , "Inoffensive, rather dull" , "painkiller" , "lacking inspiration" .)