A classical location is host to most of urban massacre? (8)
I believe the answer is:
atrocity
'massacre?' is the definition.
'a classical location is host to most of urban' is the wordplay.
'classical location' becomes 'troy'.
'is host to' means one lot of letters goes inside another.
'most of' means to remove the last letter.
'urban' becomes 'city'.
'city' with its last letter taken away is 'cit'.
'troy' going around 'cit' is 'trocity'.
'a'+'trocity'='ATROCITY'
(Other definitions for atrocity that I've seen before include "monstrous crime" , "Evil act" , "Barbarity, inhumanity" , "Heinous deed" , "Barbarous, inhuman act" .)