Mercury perhaps showing level temperature (6)
I believe the answer is:
planet
'mercury perhaps' 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 "World" , "Sun's orbiter" , "Large celestial body" , "Is Pluto one?" , "Celestial body orbiting around a star" .)