Size of a large conurbation? There's an upper limit (8)
I believe the answer is:
capacity
'size' is the definition.
(I've seen this before)
'a large conurbation? there's an upper limit' is the wordplay.
'large conurbation?' becomes 'pac' (I am not sure about this - if you are sure you should believe this answer much more).
'there's an upper' shows that the letters should be reversed in order (I've seen 'up' mean this (in down clue: letters go upwards)).
'limit' becomes 'city limit' (city limit is a kind of limit).
'a'+'pac'='apac'
'apac' reversed gives 'capa'.
'capa'+'city'='CAPACITY'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for capacity that I've seen before include "The amount that something can hold" , "The amount that can be contained, space" , "Potential - role" , "Ability, volume" , "legal competence" .)