Brush with celebrity over astral phenomenon (8)
I believe the answer is:
stardust
'astral phenomenon' is the definition.
'brush with celebrity over' is the wordplay.
'brush with' becomes 'dust' (both can mean to clean. I am not sure about the 'with' bit.).
'celebrity' becomes 'star' (I've seen this before).
'over' says to put letters next to each other.
'dust' put after 'star' is 'STARDUST'.
(Other definitions for stardust that I've seen before include "Imaginary particles blinding someone to reality" , "that could blind one to reality" , "Singer < and song" , "In Woodstock we are this" .)