Duller subject? (6)
I believe the answer is:
matter
This clue is a double definition.
'duller' is the first definition.
('matt' can be similar in meaning to 'dull')
'subject?' is the second definition.
(synonyms)
(Other definitions for matter that I've seen before include "Substance as opposed to spirit" , "Subject; be important" , "Physical substance" , "Substance; be important" , "Carry weight" .)