Whitehall monument (8)
I believe the answer is:
cenotaph
'whitehall monument' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cenotaph that I've seen before include "Tomb-like monument to people buried elsewhere" , "Empty tomb" , "Feature of Whitehall" , "construction in Whitehall" , "What divides Whitehall" .)