Entirely boring (4)
I believe the answer is:
dead
This clue is a double definition.
'entirely' is the first definition.
(synonyms)
'boring' is the second definition.
(I've seen this in other clues)
(Other definitions for dead that I've seen before include "Devoid of life" , "cold" , "Pushing up the daisies" , "Insensitive" , "Six feet under" .)