Part of building fell (5)
I believe the answer is:
floor
I believe this clue is a double definition.
'part of building' is the first definition.
('floor' is part of 'building')
'fell' is the second definition.
(I've seen this before)
(Other definitions for floor that I've seen before include "Bottom surface" , "Building's base" , "at bottom of forest" , "fell" , "American's story" .)