Surface temperature on Mars, say (6)
I believe the answer is:
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).
Can you help me to learn more?