Earth perhaps flat — first of theories (6)
I believe the answer is:
'earth perhaps' is the definition.
(earth is a kind of planet)
'flat first of theories' is the wordplay.
'flat' becomes 'plane' (I've seen this before).
'first of' indicates taking the first letters.
The first letter of 'theories' is 't'.
(Other definitions for planet that I've seen before include "Celestial body orbiting around a star" , "massive revolver" , "Eg, Mars, Venus" , "Neptune, for example" , "Orbiting body in the heavens" .)