Virtue of reduced speed in built-up area (8)
I believe the answer is:
chastity
'virtue' is the definition.
(synonyms)
'reduced speed in built-up area' is the wordplay.
'reduced speed' becomes 'hast' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'built-up area' becomes 'city'.
'hast' inserted into 'city' is 'CHASTITY'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for chastity that I've seen before include "Abstinence and purity" , "Medieval contraceptive device" , "Pure character" , "Sexual abstinence" , "Abstention from carnal activity" .)