Light from space enveloping second city on mountain (3,4)
I believe the answer is:
gas lamp
'light' is the definition.
(gas lamp is a kind of light)
'space enveloping second city on mountain' is the wordplay.
'space' becomes 'gap' (gap is a kind of space).
'enveloping' is an insertion indicator.
'second' becomes 's' (common abbreviation).
'city' becomes 'la' (Los Angeles).
'on mountain' becomes 'm' (geographical abbreviation. I am not sure about the 'on' bit.**).
's'+'la'+'m'='slam'
'gap' going around 'slam' is 'GAS-LAMP'.
'from' acts as a link.
(Other definitions for gas lamp that I've seen before include "Bright object" , "Victorian street light" , "may be illuminating" , "Old illuminator" , "lighter" .)