Satellite is flat and also square (6)
I believe the answer is:
planet
'satellite' is the definition.
(I know that satellite can be written as planet)
'flat and also square' is the wordplay.
'flat' becomes 'plane' (synonyms).
'and' means one lot of letters go next to another.
'also square' becomes 't' (a T-Square is a technical drawing instrument. I am not sure about the 'also' bit.).
'plane'+'t'='PLANET'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for planet that I've seen before include "Sun's orbiter" , "Large celestial body" , "one of seven in suite" , "Orbiting heavenly body" , "Orbiting body in the heavens" .)