Green land vacated following Space Age (7)
I believe the answer is:
emerald
'green' is the definition.
(I've seen this before)
'land vacated following space age' is the wordplay.
'vacated' means to remove the middle letters.
'following' means one lot of letters go next to another.
'space' becomes 'em' (in typography, a space as wide as a letter 'm').
'age' becomes 'era' (I've seen this before).
'land' with its middle taken out is 'ld'.
'em'+'era'='emera'
'ld' put after 'emera' is 'EMERALD'.
(Other definitions for emerald that I've seen before include "a type employed by the press" , "...re medal? (Anag)" , "Mere lad (anag.)" , "Highly-valued variety of beryl" , "city in Oz?" .)