Not much talked about, as planetary movements are? (7)
I believe the answer is:
orbital
'as planetary movements are?' is the definition.
'not much talked about' is the wordplay.
'not much' becomes 'bit'.
'talked' becomes 'oral' (both can mean spoken).
'about' is an insertion indicator.
'bit' put inside 'oral' is 'ORBITAL'.
(Other definitions for orbital that I've seen before include "Relating to planet's motion" , "Going round the world" , "M25, say" , "Gyratory" , "going round the city" .)