Plot alien object in orbit (6)

I believe the answer is:

planet

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'object in orbit' is the definition.
'planet' can be an answer for 'object' (planet is a kind of object). I'm not certain of the 'in orbit' bit.

'plot alien' is the wordplay.
'plot' becomes 'plan' (I've seen this before).
'alien' becomes 'et' (extra terrestrial).
'plan'+'et'='PLANET'

Can you help me to learn more?

(Other definitions for planet that I've seen before include "Eg, Mars, Venus" , "Substantial heavenly body" , "One orbiting star" , "Heavenly (or earthly!) body" , "Earth perhaps" .)

I've seen this clue in The Independent.

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