Eton chap designed memorial (8)
I believe the answer is:
cenotaph
'memorial' is the definition.
(cenotaph is a kind of memorial)
'eton chap designed' is the wordplay.
'designed' indicates an anagram (designed with the letters).
'eton'+'chap'='etonchap'
'etonchap' with letters rearranged gives 'CENOTAPH'.
(Other definitions for cenotaph that I've seen before include "Tomb-like monument to people buried elsewhere" , "Whitehall memorial" , "What divides Whitehall" , "construction in Whitehall" , "Feature of Whitehall" .)