Destroys what’s left of a building (5)
I believe the answer is:
ruins
'destroys' is the definition.
('ruin' can be a synonym of 'destroy')
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for ruins that I've seen before include "Damaged remains (of building)" , "dilapidated buildings" , "Broken-down remnants" , "Remains to be seen" , "Mars" .)