Genuine support for team of stars (8)
I believe the answer is:
sidereal
'of stars' is the definition.
(sidereal means relating to the stars)
'genuine support for team' is the wordplay.
'genuine' becomes 'real' (similar in meaning).
'support for' is a charade indicator (letters next to each other).
'team' becomes 'side' (both can mean a sports team).
'real' after 'side' is 'SIDEREAL'.
(Other definitions for sidereal that I've seen before include "Like stars" , "Full of stars" , "Relating to the distant stars" , "Relating to the constellations" , "Of the stars or constellations" .)