Demonstration, not heading for top of building (4)
I believe the answer is:
roof
'top of building' is the definition.
(a roof covers a building)
'demonstration not heading' is the wordplay.
'demonstration' becomes 'proof' (I've seen this in another clue).
'not heading' means to remove the first letter (remove the letter which begins or 'heads' the word).
'proof' with its initial letter taken off is 'ROOF'.
'for' is the link.
(Other definitions for roof that I've seen before include "House's top" , "Mansard is an example of one" , "Top covering of structure" , "Canopy" , "Topping structure" .)