Top conurbation accommodating a maximum number? (8)
I believe the answer is:
capacity
'maximum number?' is the definition.
(maximum capacity)
'top conurbation accommodating a' is the wordplay.
'top' becomes 'cap' (both can mean to beat or outdo).
'conurbation' becomes 'city' (I've seen this before).
'accommodating' means one lot of letters goes inside another.
'cap'+'city'='capcity'
'capcity' enclosing 'a' is 'CAPACITY'.
(Other definitions for capacity that I've seen before include "The amount that can be contained, space" , "how much can one take?" , "Amount that can be held - skill" , "The wherewithal" , "Ability to contain or accommadate" .)