Jockey a proven US star (9)
I believe the answer is:
supernova
'star' is the definition.
(I know that supernova is a type of star)
'jockey a proven us' is the wordplay.
'jockey' is an anagram indicator (the letters scramble or jockey around).
'a'+'proven'+'us'='aprovenus'
'aprovenus' anagrammed gives 'SUPERNOVA'.
(Other definitions for supernova that I've seen before include "Exploding star that becomes much brighter" , "it puts an end to a starry career" , "Star made brilliant by stellar explosion" , "Exploding star that becomes extremely luminous" , "Star that explodes and becomes extremely luminous" .)