Dull; apartment (4)
I believe the answer is:
This clue is a double definition.
'dull' is the first definition.
(similar in meaning)
'apartment' is the second definition.
(I know that apartment can be written as flat)
(Other definitions for flat that I've seen before include "Not sloping; apartment" , "Lacking interest" , "Accommodation < still" , "like some races" , "razed to the ground, having had blow-out" .)