Size of area in top conurbation (8)
I believe the answer is:
capacity
'size' is the definition.
(I've seen this before)
'area in top conurbation' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'in' means one lot of letters goes inside another.
'top' becomes 'cap' (both can mean to beat or outdo).
'conurbation' becomes 'city' (I've seen this before).
'cap'+'city'='capcity'
'a' inserted into 'capcity' is 'CAPACITY'.
'of' is the link.
(Other definitions for capacity that I've seen before include "how much can one take?" , "Size of stadium" , "The wherewithal" , "legal competence" , "Space; ability" .)