Crossword Genius

Surface temperature on Mars, say (6)

Ross

I believe the answer is:

planet

I'm a little stuck... Click here to teach me about this clue!

Here is my best explanation:

'mars say' is the definition.
(mars is a kind of planet)

'surface temperature on' is the wordplay.
'surface' becomes 'plane' (I've seen this before).
'temperature on' becomes 't' (I can't justify this - if you can you should give a lot more credence to this answer).
'plane'+'t'='PLANET'

Can you help me to learn more?