Around it, planets turn in opposite directions! (3)
I believe the answer is:
sun
'around it planets' is the definition.
I can't judge whether this defines the answer.
'turn in opposite directions' is the wordplay.
'turn' becomes 'u' (a 'U-turn' is a kind of turn).
'in' means one lot of letters goes inside another.
'opposite' says the letters should be written in reverse (I've seen 'opposite of' mean this).
'directions' becomes 'ns' (N is an example).
'ns' backwards is 'sn'.
'u' going inside 'sn' is 'SUN'.
Can you help me to learn more?
(Other definitions for sun that I've seen before include "Centre of a solar system" , "Source of light and heat" , "Helios, in Greek myth" , "Major star" , "Star round which we orbit" .)