Very bottom (5)
(Other definitions for floor that I've seen before include "American story" , "Lower limit" , "Storey of a building" , "Dumbfound" , "Knock down; storey" .)
'very bottom' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nadir that I've seen before include "deep despair" , "worst-case scenario?" , "most unsuccessful point" , "businessman formerly escaping justice" , "pits" .)