Clue: Heavenly body that rises with the sun? (7)
Answer: MERCURY
Explanation: Heavenly body that rises with the sun? (7)
Cryptic definition, with MERCURY as a thermometer – also very nearly as a planet, since it is so near the sun that its rising is never far in time from the sun's (except perhaps at times near the poles, where one might just rise when the other does not).