Lower surface of stump (5)
I believe the answer is:
floor
I believe this clue is a double definition.
'lower surface' is the first definition.
(lower surface of a building)
'stump' is the second definition.
(I've seen this before)
'of' is the link.
(Other definitions for floor that I've seen before include "Lower limit" , "Minimum (price) level" , "Base of room - storey" , "Dumbfound" , "Baffle, confound" .)