Tree trunk almost entirely hidden in earth? (6)

I believe the answer is:

planet

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'almost entirely hidden in earth?' is the definition.
'planet' can be an answer for 'earth?' (Earth is an example). I'm not sure about the 'almost entirely hidden in' bit.

'tree trunk' is the wordplay.
'tree' becomes 'plane tree' (plane is a type of tree).
'trunk' becomes 't' (this might be a standard abbreviation of which I'm unaware).
'plane'+'t'='PLANET'

Can you help me to learn more?

(Other definitions for planet that I've seen before include "Orbiting body in the heavens" , "one of seven in suite" , "Celestial body that revolves around the sun" , "Heavenly body moving round the sun" , "Eg, Mars, Venus" .)

I've seen this clue in The Independent.

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