In historical times, wonderfully green outskirts of the City (7)
I believe the answer is:
regency
'in historical times' is the definition.
The definition and answer can be both related to time as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'wonderfully green outskirts of the city' is the wordplay.
'wonderfully green' becomes 'regen' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'outskirts of' suggests removing the centre.
'city' with its middle taken out is 'cy'.
'regen'+'cy'='REGENCY'
Can you help me to learn more?
(Other definitions for regency that I've seen before include "Years from 1811 to 1820 during George III's periods of insanity" , "Period of king's incapacity" , "Period 1811-1820 in UK" , "Of a period of government, such as in Britain 1811-1820" , "historical period" .)