Reportedly, imperfection in lower surface (5)
I believe the answer is:
floor
'lower surface' is the definition.
(lower surface of a building)
'reportedly imperfection' is the wordplay.
'reportedly' shows a homophone (sound like).
'imperfection' becomes 'flaw' (flaw is a kind of imperfection).
'flaw' is a homophone of 'FLOOR'.
'in' is the link.
(Other definitions for floor that I've seen before include "Sea bottom; baffle" , "Knock down with force" , "Building's base" , "Knock down - planking" , "Base of room - storey" .)