Earth's level area included in test (9)
I believe the answer is:
planetary
'earth's' is the definition.
(I know this)
'level area included in test' is the wordplay.
'level' becomes 'plane' (synonyms).
'area' becomes 'a' (maths abbreviation).
'included in' indicates putting letters inside.
'test' becomes 'try' (synonyms).
'a' going within 'try' is 'tary'.
'plane'+'tary'='PLANETARY'
(Other definitions for planetary that I've seen before include "Of Mars and Uranus, say" , "Of any heavenly body moving round the sun" , "Relating to bodies in the solar system" .)