Planet's grand projection (5)
I believe the answer is:
globe
'planet's' is the definition.
(globe is a kind of planet)
'grand projection' is the wordplay.
'grand' becomes 'g' ('G' can be a synonym of 'grand').
'projection' becomes 'lobe' (I've seen this before).
'g'+'lobe'='GLOBE'
(Other definitions for globe that I've seen before include "Spherical shape" , "Rounded object" , "House 8 [OF SHAKESPEARE]" , "Shakespeare's theatre" , "Representation of earth" .)