Blue planet close to Mercury (6)
I believe the answer is:
earthy
'blue' is the definition.
(both can mean sexual)
'planet close to mercury' is the wordplay.
'planet' becomes 'earth' (Earth is an example).
'close to' suggests the final letters (the 'close' to the word).
The final letter of 'mercury' is 'y'.
'earth'+'y'='EARTHY'
(Other definitions for earthy that I've seen before include "Crude - vulgar" , "Coarse like the soil" , "where we live" , "Suggestive of soil" , "Gross, unrefined" .)