Mercury, say, showing level temperature (6)
I believe the answer is:
planet
'mercury say' is the definition.
(mercury is a kind of planet)
'level temperature' is the wordplay.
'level' becomes 'plane' (plane is a kind of level).
'temperature' becomes 't' (abbreviation).
'plane'+'t'='PLANET'
'showing' acts as a link.
(Other definitions for planet that I've seen before include "Eg, Mars or Pluto" , "World" , "one of seven in suite" , "Large celestial body" , "Celestial body that revolves around the sun" .)