Greenery, at first a modest innovation in late Georgian streets (8)
I believe the answer is:
gaslight
'innovation in late georgian streets' is the definition.
'greenery at first a modest' is the wordplay.
'at first' says to take the initial letters.
'modest' becomes 'slight'.
The initial letter of 'greenery' is 'g'.
'g'+'a'+'slight'='GASLIGHT'
(Other definitions for gaslight that I've seen before include "Bergman film" , "Old film" , "Old form of illumination" , "source of illumination" , "Sort of illumination" .)