Apartment level (4)
I believe the answer is:
flat
This clue is a double definition.
'apartment' is the first definition.
(I know that apartment can be written as flat)
'level' is the second definition.
(I know that level can be written as flat)
(Other definitions for flat that I've seen before include "Pied-a-terre" , "Boring - apartment" , "Level; dull" , "Level - uninteresting - accommodation" , "Prostrate" .)