The key to one story fell by design (5,4)
I believe the answer is:
floor plan
'design' is the definition.
(I've seen this before)
'the key to one story fell' is the wordplay.
I cannot really understand how this works, but
'key' could be 'f' (I've seen this in other clues) and 'f' is found within the answer.
'one' could be 'an' ('a' and 'an' can mean 'one') and 'an' is found within the answer.
This explanation may well be incorrect...
'by' is the link.
Can you help me to learn more?
(Other definitions for floor plan that I've seen before include "Design" , "map" .)