Level of a building (6)
I believe the answer is:
storey
'level of a building' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for storey that I've seen before include "Layer in tall building" , "Oyster (anag) -- level" , "Floor or level in a building" , "Tier of a building" , "section of building" .)