Earth perhaps flat - theory, originally (6)
I believe the answer is:
planet
'earth perhaps' is the definition.
(earth is a kind of planet)
'flat theory originally' is the wordplay.
'flat' becomes 'plane' (I've seen this before).
'originally' indicates taking the first letters.
The initial letter of 'theory' is 't'.
'plane'+'t'='PLANET'
(Other definitions for planet that I've seen before include "World" , "Is Pluto one?" , "Orbiting body in the heavens" , "Jupiter is one" , "One orbiting star" .)