Earth perhaps flat? Thinking initially (6)
I believe the answer is:
planet
'earth perhaps' is the definition.
(Earth is an example)
'flat? thinking initially' is the wordplay.
'flat?' becomes 'plane' ('plane' can be a synonym of 'flat').
'initially' suggests taking the first letters.
The initial letter of 'thinking' is 't'.
'plane'+'t'='PLANET'
(Other definitions for planet that I've seen before include "Eg, Mars, Venus" , "Heavenly (or earthly!) body" , "Eg, a ruby" , "Substantial heavenly body" , "Celestial body orbiting round a star" .)