Space age turned on first of Apollos (4)
I believe the answer is:
area
'space' is the definition.
(I've seen this before)
'age turned on first of apollos' is the wordplay.
'age' becomes 'era' (both can mean a long time).
'turned' says the letters should be written backwards.
'on' means one lot of letters go next to another.
'first of apollos' becomes 'a' (1st letter of 'apollos').
'era' written backwards gives 'are'.
'are'+'a'='AREA'
(Other definitions for area that I've seen before include "Expanse of a surface" , "Expanse or neighbourhood" , "Extent of a two-dimensional surface" , "Succession, order" , "environs" .)