Peak of about one million derelict houses (6)
I believe the answer is:
climax
'peak' is the definition.
(I've seen this before)
'of about one million derelict houses' is the wordplay.
'of about' becomes 'c' (I can't explain this - if you can you should believe this answer much more).
'one million' becomes 'im' (resembles cm).
'derelict' becomes 'lax' (I've seen this in another clue).
'houses' indicates putting letters inside.
'im' going inside 'lax' is 'limax'.
'c'+'limax'='CLIMAX'
(Other definitions for climax that I've seen before include "High point" , "Peak of a performance" , "Most intense point" , "Highest point" , "Culminating moment" .)