When 100 cities sprawled, millions would show self-denial (10)
I believe the answer is:
asceticism
'self-denial' is the definition.
(I know that asceticism is a type of self-denial)
'when 100 cities sprawled millions' is the wordplay.
I cannot really see how this works, but
'when' could be 'as' (eg 'when I walked' means 'as I walked') and 'as' is found in the answer.
'100' could be 'c' (Roman numeral) and 'c' is located in the answer.
an anagram of 'cities' is 'eticis' which is within the answer.
'millions' could be 'm' (abbreviation for millions) and 'm' is found in the answer.
No letters remain.
This explanation may well be incorrect...
'would show' acts as a link.
Can you help me to learn more?
(Other definitions for asceticism that I've seen before include "austere way of life?" , "austere lifestyle" , "Austerity" , "self-denial" .)